Rakib can complete a task in 30 minutes and together with his brother Momin, he can complete the task in 20 minutes. How long would it take for Momin working alone to complete the task?

in 30 minutes, Rakib can complete 1 task

in 1 minute, Rakib can complete 1/30 task

Again,

in 20 minutes, Rakib and Monin can complete 1 task

in 1 minute, Rakib and Monin can complete 1/20 task

Now,

let, monin does x task in 1 minute.

Now,

1/30 + 1/x = 1/20

(x + 30)/30x = 1/20

30x = 20x + 600

30x - 20x = 600

10x = 600

x = 60

Thus,

momin working alone can complete the task in 60 minutes.