39 times 50 () 2000 62 times 31 () 1800 52 times 29 () 30 times 52 19 times 38 () 20 times 40 The brackets must be greater than, less than or equal to

39 times 50 () 2000 62 times 31 () 1800 52 times 29 () 30 times 52 19 times 38 () 20 times 40 The brackets must be greater than, less than or equal to

39 times 50 (< 2000)
62 times 31 (>) 1800
52 by 29（

24, 28, 36, 52

4×6 4×7 4×9 4×13
The next one is 4 × 21 = 84
It's the same before the multiply sign, but it's the same after the multiply sign

Find a rule to fill in the blanks and calculate: 54 + 37 = 91, 53 + 38 = 91, 52 + 39 = 91, 51 + 40 = 91, one addend is smaller than 1 each time, the other addend is larger than 1 each time, and how much? Please tell me how to solve the problem

In the end, there is no change, so the sum is still 91