diff --git a/interfaces/08-promoted-methods/list.go b/interfaces/08-promoted-methods/list.go
index 381de50..f1a6ceb 100644
--- a/interfaces/08-promoted-methods/list.go
+++ b/interfaces/08-promoted-methods/list.go
@@ -11,7 +11,7 @@ import "fmt"
 
 type item interface {
 	print()
-	discount(float64)
+	discount(ratio float64)
 }
 
 type list []item
diff --git a/interfaces/09-stringer/list.go b/interfaces/09-stringer/list.go
index 7b48878..d640d7e 100644
--- a/interfaces/09-stringer/list.go
+++ b/interfaces/09-stringer/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 
 	// item interface embeds the fmt.Stringer interface.
 	//
diff --git a/interfaces/10-marshaler/list.go b/interfaces/10-marshaler/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/10-marshaler/list.go
+++ b/interfaces/10-marshaler/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/11-decode-toiface/list.go b/interfaces/11-decode-toiface/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/11-decode-toiface/list.go
+++ b/interfaces/11-decode-toiface/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/12-reflection/list.go b/interfaces/12-reflection/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/12-reflection/list.go
+++ b/interfaces/12-reflection/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/13-reflection-2/list.go b/interfaces/13-reflection-2/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/13-reflection-2/list.go
+++ b/interfaces/13-reflection-2/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/14-io-reader/list.go b/interfaces/14-io-reader/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/14-io-reader/list.go
+++ b/interfaces/14-io-reader/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/15-io-reader-composition/list.go b/interfaces/15-io-reader-composition/list.go
index 69da963..01b6eaa 100644
--- a/interfaces/15-io-reader-composition/list.go
+++ b/interfaces/15-io-reader-composition/list.go
@@ -13,7 +13,7 @@ import (
 )
 
 type item interface {
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer
 }
 
diff --git a/interfaces/composition/list.go b/interfaces/composition/list.go
index a89abc8..39ab0c7 100644
--- a/interfaces/composition/list.go
+++ b/interfaces/composition/list.go
@@ -14,7 +14,7 @@ import (
 
 type item interface {
 	sum() money
-	discount(float64)
+	discount(ratio float64)
 	fmt.Stringer // same as: `String() string`
 }