There is provision of food in fort for 1200 soldiers for 60 days. After 15 days, 200 soldiers leave the fort. Remaining food will last for how many days?

Answer: Option C

Work equivalence method:
1200 × 45 = 1000 × x
Hence, x = 54 days

Variation Method:
After 15 days 200 soldiers leaved.
Soldiers              Food for days
1200 ↓                 45
1000                 ↑ x (let)
Arrows show the opposite variation to each other
$$\frac{1200}{1000}=\frac{\text{x}}{45}$$
Or, x = $$\frac{1200\times 45}{1000}$$   = 54 days.