Explanation

The 20 men working 5 days contribute 20 × 5 = 100 man-days, which equals one-fourth of the work, so the whole work is 400 man-days. The remaining 300 man-days must be done in the remaining 5 days, requiring 300 ÷ 5 = 60 men per day. With 20 men already present, 60 − 20 = 40 more men are needed. This is option (a).