The rule of the second question is that the sum of one of the left and right sides is equal to the product of two numbers on the other side, so three spaces are filled with 6 and four spaces are filled with 24.
The rule of the third question (in my opinion) is 2+4=6 3+6=9 4+7= 1 1 less than the above number, so the fourth question should be filled in 1 5.