It’s c got perfect from this
According to the information presented, Benny should decide not to add a second chair because the average number of clients waiting is 1 (very low), the average waiting time for a client is 10 minutes (very low), and the average time a customer is in the store is 20 minutes (very low).
According to the information, an average of 2 clients arrive at the hairdresser for every hour (60 min) and each cut takes a maximum of 20 minutes. Therefore, the hairdresser would have an average of 40 minutes busy and 20 minutes free for each hour.
Additionally, customers who must wait do not have to spend a long time waiting in the store because the maximum waiting time is 10 minutes (in the case that both customers arrive at the same time).
Below is a graph where the information of both cases is compared (with an additional chair and without an additional chair) and it is inferred that an additional chair is not necessary because the number of clients waiting and the waiting time is not high.
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Actually, the answer is not A, if you're saying A is the first choice above. That's incorrect. You will need to use the Geometric mean for right triangles here to figure out what the value of a is. We will use this form:

. We have a value for YZ of 3; side a is XZ. That means in order to solve this we need WZ, which we can find using pythagorean's theorem. 3^2 + 4^2 = c^2 and 9 + 16 = c^2 and c = 5. Now we fill in accordingly:

. Cross-multiply to get 3XZ=25 and side XZ is

. XZ is 25/3 and YZ is 3, so 25/3 - 3 = XY. That means that XY (side a) = 16/3 or 5 1/3, choice B, or the second one down.