Question

Cookies are sold singly or in packages of 10 or 30. With this​ packaging, how many ways can you buy 60 ​cookies?

There is an answer on here but it is saying it is wrong.

Answer 1

for easier, let's say x represents a pack of 10 and y represents a pack of 30

we can start with 2 of the 30's
2y=60
we can replace one of the 30's with 3 of the 10's
y+3x=60

and finally we can use all 10's
6x=60

there are only 3 ways