To produce n bolts, Machine A takes 2 hours more than Machine B does.

Since this is a rate question with every result related to n, we can divide all quantities of bolts by n and it won't change anything. Thus to produce 1 bolt, Machine A takes 2 hours more than Machine B does etc. Set the rate of A as \(\frac{1}{x + 2}\) and rate of B as \(\frac{1}{x}\).

Then we have: \(\frac{3}{x + 2} + \frac{3}{x} = \frac{5}{4}\)

Here we can try plugging in numbers from the choices, the given choices in terms of x are \(2(x + 2)\), however, so we need to divide by 2 first then plugin for x + 2.

Start from C, we plug in x + 2 = 5 and get \(\frac{3}{5} + \frac{3}{3} = \frac{8}{5}\), that result is too big so we need a bigger x to reduce the result.

Next we try D, which gives \(\frac{3}{6} + \frac{3}{4} = \frac{5}{4}\).

Ans: D