In order to figure out how much sunscreen he has EXACTLY we can use the expression 12 + .2(12), which is the 12 ounces regular size plus 20% of the regular size. Doing that math, we get that the family size is 14.4 ounces. 4 bottles, then, would be 14.4 * 4 which is an even 72 ounces. So yes, he has at least 50 ounces, but his estimation is quite a bit off.
