What was the question for this implicit rationale, and corresponding answer?
Mike is using 40 blocks / 2 = 20 blocks of type B. He uses in total 40 blocks + 20 blocks = 60 blocks of type A and B. So he is going to use 150 blocks - 60 blocks = 90 blocks of other types.
 The answer: 90
The question: Mike is building a bridge out of LEGO blocks. To be successful he needs at least 40 bricks of type A, and half that many of type B. In total, he needs to use 150 bricks. How many bricks of other types than mentioned is he going to use?