Three ways to solve:
-- If adding 4 part-time workers changes the ratio from "2 to 5" to "3 to 5", then 4 must represent one part of the ratio, and since we have 7 parts in the original ratio, we have 28 workers.
-- If the original ratio is 2 to 5, the total must be divisible by 2 + 5 = 7, so C, D or E is right. If we get a 3 to 5 ratio by hiring 4 workers, then we must get a multiple of 3 + 5 = 8 when we add 4 to the right answer, and D must be right.
-- If the ratio of part-time to full-time workers is initially 2 to 5, we must have 2x part-time workers, and 5x full-time workers, for some integer x, and the question asks us to find 2x + 5x = 7x. When we hire 4 new part-time workers, we have 2x + 4 part time workers, and we still have 5x full-time workers. These are in a 3 to 5 ratio, so
(2x + 4)/5x = 3/5
10x + 20 = 15x
x = 4
and 7x, which we wanted to find, is 28.
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