Explanation:

Let’s break it down:

- A works twice as fast as B, and B works twice as fast as C.

- So, if C does 1 unit/day, B does 2 units/day, and A does 4 units/day.

- Together, A+B+C = 4+2+1 = 7 units/day.

- D alone finishes the job in 16 days, so D does 1/16 of the work per day.

- All four together finish in 4 days.

- So, (A+B+C+D) × 4 = 1 whole job.

- That’s (7 + 1/16) × 4 = 1.

- Let’s calculate:

- 7 + 1/16 = 113/16 units/day.

- In 4 days: (113/16) × 4 = 113/4 units.

- This much work must equal the whole job (which is 1), but the calculation shows that actually (A+B+C+D) = 1/4 units/day.

- Actually, (A+B+C+D) × 4 = 1

So (A+B+C+D) = 1/4 per day.

- We already have D = 1/16 per day.

- So, (A+B+C) = 1/4 – 1/16 = (4-1)/16 = 3/16 per day.

- A+B+C = 3/16 per day.

We also know A:B:C = 4:2:1, so total shares = 7.

So A = 4/7 × (3/16) = 12/112 = 3/28 per day.

- To finish 75% of the job:

- Time = (3/4)/(3/28) = (3/4) × (28/3) = 28/4 = 7 days.

Here’s what’s up with the options:

- 1, 8 – Nope

- 2, 7 – Yes, that’s exactly what we calculated

- 3, 9 – No

- 4, 6 – No