What number should be subtracted from each of the number 54, 71, 75 and 99 so that the remainders may be proportional?

Let the required number be N,
From question, (54 - N) / (71 - N) = (75 - N) / (99 - N)
⇒ (54 - N) x (99 - N) = (75 - N) x (71 - N)
⇒ N² - 153N + 5346 = N² - 146N + 5325
⇒ 7N = 21
∴ N = 3