It took 12 minutes to cut a log into 3 pieces. How many minutes will it take to cut it into 4 pieces. The answer is not 16.

How many times do you cut in order to make three pieces?

This is a variation on what is called the fence post problem. If you are building a 100' fence with posts 5' apart, you have 20 spaces but 21 posts.
Sort of conversely with this problem, you have 3 pieces but only made 2 cuts. So how long did each of those take?

answer is 18...

took u 12 minutes to make 2 cuts (3 pieces)
so, for 3 cuts (4 pieces) the solutions is (3*12)/2 = 18 minutes,,