well to get percent you the equation is
(amount of specific) / (total amount) * 100% so:
13 / 20 * 100% = 65%
So if 60% of all cans are recycled now you can use that as your ratio to get how many are recycled in any given total amount by multiplying it by that total and dividing by 100% so:
(65% * 50) / 100% = 32.5
There is no 0.5 can amount so i would round to 33 but the question or teacher will always clarify what to do