An army’s recruitment process included n rounds of selection tasks. For the first a rounds, the rejection percentage was 60 percent per round. For the next b rounds, the rejection percentage was 50 percent per round and for the remaining rounds, the selection percentage was 70 percent per round. If there were 100,000 people who applied for the army and 1400 were finally selected, what was the value of n?
For the three sets of rounds, 100,000 is multiplied by 0.40, 0.50, and 0.70.
One way to find the total number of rounds is to work in from the ends and see what has to happen to get from 100,000 to 1,400.
So, at the top end, we can multiply 100,000 by 0.40 to get 40,000.
At the bottom end, we can work backward by dividing 1,400 by 0.70 to get 2,000.
Having done so, we can see that there's no way to get to 2,000 from 40,000 by multiplying by 0.50 or 0.70 since dividing 40,000 by 2 three times will leave us with 5,000 and since 2,000 is not a multiple of 7.
So, we can tell that we have to multiply by 0.40 one more time.
Doing so, we get 40,000 × 0.40 = 16,000.
16,000/8 = 2,000
So, we can now multiply 16,000 by 0.50 three times to get to 2,000.
Thus, we have the following:
100,000 × 0.40^2 = 16,000
16,000 × 0.50^3 = 2,000
2000 × 0.70 = 1,400
So, we have a total of 6 rounds.
(A) 4
(B) 5
(C) 6
(D) 8
(E) 10
Correct answer: C
Another way find the answer is to divide 1,400 by 100,000 to get 0.014 and then determine how many times 0.4, 0.5, and 0.7 go into 0.014.
0.014 = 0.7 × 0.2 × 10^-1 = 0.7 × 0.5 × 0.4 × 10^-1 = 0.7 × 0.5 × 0.4 × 0.5 × 0.2 = 0.7 × 0.5 × 0.4 × 0.5 × 0.5 × 0.4
So, we have a total of 6 rounds.
(A) 4
(B) 5
(C) 6
(D) 8
(E) 10
Correct answer: C