Answer:
About 900 cans not stamped in 3 hours
Explanation:
Proportions
Sally found that 150 cans were not stamped in the 1/2 hour trial run.
Following the same proportion in time, we can say:
150 cans not stamped in 1/2 hour. Multiplying by 2:
300 cans not stamped in 1 hour. Multiplying by 3:
900 cans not stamped in 3 hours
About 900 cans not stamped in 3 hours