Explanation:
so, working together, in 1 day 1/10 + 1/15 of the work is done.
how many days (x) until we get 1/1 of the work done ?
x × (1/10 + 1/15) = 1
x × (3/30 + 2/30) = 1
x × 5/30 = 1
x×5 = 30
x = 30/5 = 6
so, together, they will finish the work in 6 days.