On the playground the teacher observed bicycles and tricycles. she counted 72 wheels and 28 total cycles. How many of them were bicycles and how many of them were tricycles?

16 tricycles

12 bicycles

Step-by-step explanation:

If all were bicycles, there would be 28·2 = 56 wheels. Each additional wheel indicates the presence of a tricycle instead of a bicycle.

There are 72 - 56 = 16 tricycles. The remaining 28-16 = 12 are bicycles.