Penny and kenny have the same number of football cards.

penny has sorted her cards into piles of 10.

kenny has sorted his cards into piles of 18.

penny has less than 100 cards.

how many football cards do they have?

Hello,
Penny==>a*10 (a is an integer)
Kenny==>b*18 (b is an integer)

a*10=b*18==>a=9*b/5 ==>b=5+n*5
a=9*(5+5*n)/5=9*(1+n)=9+9n
If n=0 then a=9 and b=5 :Number od cards: 9*10=10, 5*18=90
if n=1 then a=19 and b=10: Number of cards: 19*10=190>100 to exclude.

So number of cards=90