Suppose our son can drink for x days.

1/25+ 1/x = 1/20

1/x = 1/20- 1/25

1/x = 5/ 100-4/ 100

1/x= 1/ 100

X= 100 (days)

Method 2:

1÷( 1/20- 1/25)

= 1÷ 1/ 100

= 100 (days)

Remember to adopt it as the best.