Question

Two dads and two son's went to an ice cream shop and they all bought one each it cost's 50 cents but they only paid $1.50

Answer 1

There were only 3 men.

One was the grandfather aka the father #1

One was the father to the child and simultaneously the son of the child's grandfather (father #2 and son #1)

One was the child (child #2)

Answer 2

If there were two dad's and two sons, and they each bought an ice cream cone worth 50, but only paid 1.50, that means there was a grandpa, who is a dad,

a dad, who is a dad of the child and a son of the grandpa,

and a son, who is a son of the dad.

This all adds up to two dads and two sons.

(the dad is both a dad and a son, if you don't understand ;))