1. Because the sum of 5+3+9= 17 and 5+3+a+9+b is even, and a+b is odd. So the parity of a and b is different.

2. You should learn to set the unknown X in the fifth grade.

Suppose it takes X days for them to meet, then X is also an integer multiple of 2.3.4, so X is the least common multiple of 2.3.4 2*3*4=24 days.

answer ....

This should be the same in primary school.