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diff --git a/004-mutation/mutation.go b/004-mutation/mutation.go
deleted file mode 100644
index e554ba2..0000000
--- a/004-mutation/mutation.go
+++ /dev/null
@@ -1,16 +0,0 @@
-// ## Mutation
-
-package main
-
-import "fmt"
-
-func main() {
-    var x string
-    x = "old"
-    fmt.Println(x)
-
-    // You can mutate variables in Go by re-assigning the
-    // variable to a new value.
-    x = "new"
-    fmt.Println(x)
-}
diff --git a/004-mutation/mutation.sh b/004-mutation/mutation.sh
deleted file mode 100644
index 97fa82e..0000000
--- a/004-mutation/mutation.sh
+++ /dev/null
@@ -1,3 +0,0 @@
-$ go run mutation.go
-old
-new
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index 60affa0..0000000
--- a/011-sum/sum.go
+++ /dev/null
@@ -1,20 +0,0 @@
-// ## Sum
-
-package main
-
-import "fmt"
-
-func main() {
-    var x [5]float64
-    x[0] = 98
-    x[1] = 93
-    x[2] = 77
-    x[3] = 82
-    x[4] = 83
-    
-    var total float64 = 0
-    for i := 0; i < 5; i++ {
-        total += x[i]
-    }
-    fmt.Println(total / float64(len(x)))
-}
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--- a/013-floats/floats.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// ## Floats
-
-package main
-
-import "fmt"
-
-func main() {
-	x := [5]float64{ 98, 93, 77, 82, 83 }
-	total := 0.0
-	for _, v := range x {
-		total += v
-	}
-	fmt.Println(total / float64(len(x)))
-}
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+++ b/079-sending-email/sending-email.go
@@ -1,4 +1,4 @@
-// ## Email
+// ## Sending Email
 
 package main
 
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rename to 082-http-server-graceful-shutdown/http-server-graceful-shutdown.go
diff --git a/085-http-server-graceful-shutdown/http-server-graceful-shutdown.sh b/082-http-server-graceful-shutdown/http-server-graceful-shutdown.sh
similarity index 100%
rename from 085-http-server-graceful-shutdown/http-server-graceful-shutdown.sh
rename to 082-http-server-graceful-shutdown/http-server-graceful-shutdown.sh
diff --git a/086-http-server-log/http-server-log.go b/083-http-server-log/http-server-log.go
similarity index 100%
rename from 086-http-server-log/http-server-log.go
rename to 083-http-server-log/http-server-log.go
diff --git a/087-http-server-middleware/http-server-middleware.go b/084-http-server-middleware/http-server-middleware.go
similarity index 100%
rename from 087-http-server-middleware/http-server-middleware.go
rename to 084-http-server-middleware/http-server-middleware.go
diff --git a/088-http-server-routing/http-server-routing.go b/085-http-server-routing/http-server-routing.go
similarity index 100%
rename from 088-http-server-routing/http-server-routing.go
rename to 085-http-server-routing/http-server-routing.go
diff --git a/088-http-server-routing/http-server-routing.sh b/085-http-server-routing/http-server-routing.sh
similarity index 100%
rename from 088-http-server-routing/http-server-routing.sh
rename to 085-http-server-routing/http-server-routing.sh
diff --git a/089-http-server-static-dynamic/http-server-static-dynamic.go b/086-http-server-static-dynamic/http-server-static-dynamic.go
similarity index 100%
rename from 089-http-server-static-dynamic/http-server-static-dynamic.go
rename to 086-http-server-static-dynamic/http-server-static-dynamic.go
diff --git a/089-http-server-static-dynamic/http-server-static-dynamic.sh b/086-http-server-static-dynamic/http-server-static-dynamic.sh
similarity index 100%
rename from 089-http-server-static-dynamic/http-server-static-dynamic.sh
rename to 086-http-server-static-dynamic/http-server-static-dynamic.sh
diff --git a/090-http-server-static-select/http-server-static-select.go b/087-http-server-static-select/http-server-static-select.go
similarity index 100%
rename from 090-http-server-static-select/http-server-static-select.go
rename to 087-http-server-static-select/http-server-static-select.go
diff --git a/090-http-server-static-select/http-server-static-select.sh b/087-http-server-static-select/http-server-static-select.sh
similarity index 100%
rename from 090-http-server-static-select/http-server-static-select.sh
rename to 087-http-server-static-select/http-server-static-select.sh
diff --git a/091-http-server-static/http-server-static.go b/088-http-server-static/http-server-static.go
similarity index 100%
rename from 091-http-server-static/http-server-static.go
rename to 088-http-server-static/http-server-static.go
diff --git a/091-http-server-static/http-server-static.sh b/088-http-server-static/http-server-static.sh
similarity index 100%
rename from 091-http-server-static/http-server-static.sh
rename to 088-http-server-static/http-server-static.sh
diff --git a/092-http-server-status-code/http-server-status-code.go b/089-http-server-status-code/http-server-status-code.go
similarity index 100%
rename from 092-http-server-status-code/http-server-status-code.go
rename to 089-http-server-status-code/http-server-status-code.go
diff --git a/092-http-server-status-code/http-server-status-code.sh b/089-http-server-status-code/http-server-status-code.sh
similarity index 100%
rename from 092-http-server-status-code/http-server-status-code.sh
rename to 089-http-server-status-code/http-server-status-code.sh
diff --git a/093-http-server/http-server.go b/090-http-server/http-server.go
similarity index 100%
rename from 093-http-server/http-server.go
rename to 090-http-server/http-server.go
diff --git a/094-https-server/https-server.go b/091-https-server/https-server.go
similarity index 100%
rename from 094-https-server/https-server.go
rename to 091-https-server/https-server.go
diff --git a/094-https-server/https-server.sh b/091-https-server/https-server.sh
similarity index 100%
rename from 094-https-server/https-server.sh
rename to 091-https-server/https-server.sh
diff --git a/tool/index.txt b/tool/index.txt
index fe2b6ca..b09e77b 100644
--- a/tool/index.txt
+++ b/tool/index.txt
@@ -6,16 +6,13 @@
 hello-world
 values
 variables
-mutation
 literals
 constants
 for
 if-else
 case
 arrays
-sum
 range
-floats
 slices
 maps
 nesting
@@ -100,7 +97,7 @@ https-client
 redis
 postgres
 ~ mongodb
-email
+sending-email
 
 # web apps
 http-server-basic