Try thinking about how many houses 1 maid can clean in 1 day.

If 5 maids can clean 7 houses in 3 days, then 1 maid can clean 7/5 house in 3 days and 7/15 house in 1 day.

(8)7/15=56/15. This is how many houses 8 maids can clean in 1 day.

How many days then for 20 houses.

20(15/56)=75/14

It'll take 8 maids 75/14 days to clean 20 houses.

It may be easier to look at the proportions:

(5 maids)(3 day)=7 houses

(8 maids)(x days) = 20 houses

7/15=20/(8x)

Solve for x = 75/14