In order to efficiently bid a contract, a contractor wants to be 95% confident that his error

Question:

In order to efficiently bid a contract, a contractor wants to be 95% confident that his error is less than two hours in estimating the average time it takes to install tile flooring. Previous contracts indicate that the standard deviation is 4.5 hours. How large a sample must be selected? The is also what I have come up with (1.96*4.5/2)x2=1.9448=19.45