Answer:
74.0 lb
Explanation:
Since 15% melts in 1 hour, after 1 hour, 85% of 520 lb is left.
After each subsequent hour, 85% of the previous weight is left.
Original weight: 520 lb
After 1 hour: 520 lb × 0.85
After 2 hours: 520 lb × 0.85 × 0.85 = 520 × 0.85²
After 3 hours: 520 lb × 0.85² × 0.85 = 520 lb × 0.85³
...
After n hours: 520 lb × 0.85^n
Here, we need the weight after 12 hours, so n = 12.
weight after 12 hours = 520 lb × 0.85^12
weight after 12 hours = 73.96569... lb
Answer: 74.0 lb