Related facts and missing numbers

Answer: 50

1. Addition and subtraction undo each other.
2. Subtract the known part from the whole: 81−31=50.

Answer: 100

1. The same fact applies to tens.
2. 6 tens +4 tens =10 tens =100.

Answer: 159

1. Undo subtracting 28 by adding 28.
2. 131+28=159, so the starting number is 159.