well, there obviously is 32 bikes.
and there has to be at least two wheels on each bike.
so that is 64 wheels gone there.
72 minus 64 is 8.
so with all the bikes already having wheels on them, just add one more wheel to 8 bikes and you have your answer.
32 minus 8 is 24.
24 bicyles and 8 tricyles.
24(bicyles) times two is 48(wheels)
and
8(tricyles) times 3 is 24(wheels)
48 plus 24 is 72