Use int extensions.

app/src/main/java/me/zobrist/tichucounter/Extensions.kt

@@ -0,0 +1,10 @@
+package me.zobrist.tichucounter
+
+fun Int.isMultipleOf5(): Boolean
+{
+    return (this % 5) == 0
+}
+
+fun Int.isMultipleOf100(): Boolean {
+    return (this % 100) == 0
+}

app/src/main/java/me/zobrist/tichucounter/History.kt

@@ -1,8 +1,5 @@
-@file:Suppress("unused")
-
 package me.zobrist.tichucounter
 
-
 class History {
     private var scores: ArrayList<Round> = ArrayList()
 

app/src/main/java/me/zobrist/tichucounter/Round.kt

@@ -22,17 +22,13 @@ class Round() : Serializable {
     }
 
     private fun calculateOtherScore(score: Int): Int {
-        if (isMultipleOf100(score)) {
+        if (score.isMultipleOf100() && score != 0) {
             return 0
         }
         return 100 - (score % 100)
     }
 
-    private fun isMultipleOf100(score: Int): Boolean {
-        return (score / 100) >= 1 && (score % 100) == 0
-    }
-
     fun isValidRound(): Boolean {
-        return (scoreA % 5 == 0) && (scoreB % 5 == 0) && ((scoreA + scoreB) % 100 == 0)
+        return (scoreA.isMultipleOf5()) && scoreB.isMultipleOf5() && (scoreA + scoreB).isMultipleOf100()
     }
 }