4 men and 5 women can complete a work in 15 days, whereas 9 men and 6 women can do it in 10 days. To complete the same work in 7 days, how many women should assist 4 men?

Answer: Option D

(4M + 5W) × 15 = (9M + 6W) × 10
12M + 15W = 18M + 12W
$$\frac{\text{M}}{\text{W}}=\frac{1}{2}$$
(4M + xW) × 7 = (4 × 1 + 5 × 21 × 15) = Total work
(4 × 1 + 2x) × 7 = 14 × 15
2x = 26