S can finish 50% of a work in ‘a’ days. T can do 25% of the work in ‘a’ day. Both of them together will finish the work in ____ days. A. 2.66 B. 2.33 C. 1.33 D. 1.67

S does 50% in a days, so S alone needs 2a days; T does 25% in a days, so T alone needs 4a days. Together they do 1/(2a) + 1/(4a) = 3/(4a) of the work per day, so they need 4a/3 ≈ 1.33a days, i.e. 1.33 days when a = 1.