Lita can complete the project in 15 days, Babita can complete the same project…

Concept: When two people work on the same task, each person's work rate is 1 ÷ (days needed alone). Working together, their daily rates simply add, and the time to finish together is the reciprocal of that combined rate.

Application:

1. Lita's work rate = 1/15 of the project per day (since she alone takes 15 days).

2. Babita's work rate = 1/20 of the project per day (since she alone takes 20 days).

3. Combined daily rate = 1/15 + 1/20 = 4/60 + 3/60 = 7/60 of the project per day.

4. Time needed together = 1 ÷ (7/60) = 60/7 days.

Cross-check: In 60/7 days at a combined rate of 7/60 per day, the work done = (60/7) × (7/60) = 1, i.e. the whole project — confirming the value. This also fits the expected range: since Lita (the faster worker) alone needs 15 days, two people working together must take somewhere between half that (7.5 days) and the full 15 days; 60/7 ≈ 8.57 days sits inside that range.

So Lita and Babita together complete the project in 60/7 days.