If 12.5% of p is equal to 50% of q, what is the ratio p : q?
- (a)1 : 4
- (b)4 : 1
- (c)1 : 2
- (d)2 : 1
Answer
Why
Correct — B.
Write the percentages as fractions: 12.5% = 1⁄8 and 50% = 1⁄2
Set them equal: p⁄8 = q⁄2
Multiply both sides by 8: p = 4q
Divide both sides by q: p⁄q = 4⁄1
So p : q = 4 : 1 → option (b)
Why the others are wrong
- (a)1 : 4 — This is q : p, the ratio turned upside down. Test it: p = 1 and q = 4 give 12.5% of 1 = 0.125 against 50% of 4 = 2.
- (c)1 : 2 — p = 1, q = 2 gives 12.5% of 1 = 0.125 against 50% of 2 = 1. The two sides differ by a factor of 8.
- (d)2 : 1 — p = 2, q = 1 gives 0.25 against 0.5, so the sides still differ by a factor of 2. It is half the true ratio 4 : 1.
Concept
x% of A = y% of B means xA = yB, so A : B = y : x. The percentages swap sides.
Here x = 12.5 and y = 50, so p : q = 50 : 12.5 = 4 : 1.
It makes sense: p needs only a small slice (12.5%) to equal half of q, so p must be the bigger number.
Key facts
- x% of A = y% of B gives A : B = y : x
- 12.5% = 1⁄8, so 12.5% of p is p⁄8
- The quantity taken at the smaller percentage is the larger quantity
Study next
Common traps
- Copying the percentages in the order they appear, 12.5 : 50, and answering 1 : 4
18 Sep 2025, 12:30, Quant Q.1 is the same form: 30% of m = 18% of n gives m : n = 18 : 30 = 3 : 5.
19 Sep 2025, 09:00, Quant Q.2 stretches it to three terms, 25% of A = 20% of B = 10% of C, keyed 4 : 5 : 10.
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