Assumption method


Question says that there are 111 wheels, and 43 bicycles and tricycles.

Assume that all 43 are bicycles first.

Number of wheels = 43 x 2 = 86

When you remove a bicycle , your subtract 2 wheels from the total (-2). When you add a tricycle, you add 3 wheels to the total (+3).

So when one swaps a bicycle for tricycle,
-2 + 3 = +1

There is an increase of 1 wheel.

Now we know total number of wheels
= 111

111 - 86 = 25

So this is as good as saying we swapped 25 bicycles for tricycles.

It also tells you that there are actually 25 tricycles