The second question is 4 and 5, because the reciprocal of two natural numbers will inevitably turn 9/20 into a natural number, so 9 can be divisible by the sum of two integers and 20. Only 4/5 is suitable for exclusion (calculated according to my understanding).

The third question is because XY=25/2 because 1/xy= 12.5.

. . 1/x-1/y = 2. 5 y-x/xy = 2. So y-x=5.

So y=5+X, take y in and calculate slowly.

Fourth question: How old are you?