25 men can finish a piece of work in 20 days working 7 hours per day. In how many days a group of 21 men will finish the work when they are working 5 hours per day if 2 men of the first group are equal to 3 men of the second group ?

50

using formula :-

M₁ × D₁ × H₁ × E₁   = M₂ × D₂ × H₂  × E₂

ratio of efficiency    E₁   :   E₂

                             3     :     2

25 × 20 × 7 × 3 =   D₂  ×  21 × 5 × 2

D₂ =50