Explanation:

Correct option 4:30
Given
6 men and 10 boys, as well as 2 men and 20 boys, can finish the same work in 12 days.
*Total work = Number of workers x Efficiency of each worker x Total time taken
Let the efficiency of a man and a boy be M and B respectively.
According to the question,
(6 x M + 10 x B) x 12 = (2 x M + 20 x B) x 12
= 2M = 5B
Hence, considering total work = (6M+ 10B) x 12 = (15B + 10B) x 12 = 300B units (: 2M = 5B)
Thus, time taken by 10 boys to finish the same work = 300B = 10B = 30 days