Explanation:

Let, total work is 20 (took lcm of 10 & 20),

Atul can complete in 10 days = 20/10 = 2 work per day,

Rekha can complete in 20 days = 20/20 = 1 work per day,

Both, can complete 3 woks per day, total work is 20, they worked for 5 days = 5 x3 = 15 work done,

remaining work 5 or 5/20 = 1/4.

Hence, option 3 is the correct answer.