Kate bought 7 packs × 18 balls / pack = 126 red bouncy balls. She also bought 6 packs × 18 balls / pack = 108 yellow bouncy balls. Kate bought 126 balls - 108 balls = 18 more red bouncy balls than yellow bouncy balls.
This justifies what answer for what question? Q & A:
Kate bought 7 packs of red bouncy balls and 6 packs of yellow bouncy balls. Each pack contained 18 bouncy balls. How many more red bouncy balls than yellow bouncy balls did Kate buy?
18