What are the sides lengths of the rectangle perimeter 40in

We can't tell. The perimeter doesn't tell you the side lengths. All we know is that the length of the rectangle plus its width add up to 20 inches, but they could be anything.

There are an infinite number of different lengths and widths that it could have. Here are a few examples:

1 inch by 19 inches

2 inches by 18 inches

3 inches by 17 inches

4 inches by 16 inches

5 inches by 15 inches

6 inches by 14 inches

7 inches by 13 inches

8 inches by 12 inches

9 inches by 11 inches

9.1 inches by 10.9 inches

10 inches by 10 inches

.

.

etc.

The sides of a rectangle with the perimeter of 40 inches could be

1 by 19

2 by 18

3 by 17

4 by 16

5 by 15

6 by 14

7 by 13

8 by 12

9 by 11

10 by 10

and any of the opposites. The bolded ones are most common/most likely.