Shortcut Trick
If x is subtracted from a, b, c, d to make them proportional, then x = (ad − bc) ÷ ((a + d) − (b + c)).
Here, a = 23, b = 39, c = 32, d = 56.
x = (23 × 56 − 39 × 32) ÷ ((23 + 56) − (39 + 32)) = (1288 − 1248) ÷ (79 − 71) = 40 ÷ 8 = 5.
First number = x + 4 = 5 + 4 = 9, Second number = 3x + 1 = 3(5) + 1 = 16.
Mean proportional = √(9 × 16) = √144 = 12.
∴ The correct answer is 12.
Alternate Method
Given:
Numbers: 23, 39, 32, and 56.
x is subtracted from each number to make them proportional.
We need to find the mean proportional between (x + 4) and (3x + 1).
23 − x
39 − x
=
32 − x
56 − x
Mean Proportional = √(9 × 16) = 12
Formula Used:
If four numbers a, b, c, d are in proportion, then a ÷ b = c ÷ d.
Mean proportional between p and q = √(p × q)
Calculations:
⇒ (23 − x) ÷ (39 − x) = (32 − x) ÷ (56 − x)
⇒ (23 − x)(56 − x) = (32 − x)(39 − x)
⇒ 1288 − 56x − 23x + x2 = 1248 − 39x − 32x + x2
⇒ 1288 − 79x = 1248 − 71x
⇒ 1288 − 1248 = 79x − 71x
⇒ 40 = 8x
⇒ x = 5
Now, substitute x = 5 in the expressions (x + 4) and (3x + 1):
⇒ x + 4 = 5 + 4 = 9
⇒ 3x + 1 = 3(5) + 1 = 16
⇒ Mean proportional = √(9 × 16)
⇒ Mean proportional = √144 = 12
∴ The correct answer is 12.
Additional Information
Mean Proportional
For two positive numbers a and b, the mean proportional is the square root of their product: √(a × b).
Third Proportional
For two given numbers a and b, the third proportional c satisfies the relation a : b = b : c, giving c = b2 ÷ a.
Fourth Proportional
For three numbers a, b, and c, the fourth proportional d satisfies a : b = c : d, which simplifies to d = (b × c) ÷ a.
Componendo and Dividendo
If a ÷ b = c ÷ d, then according to this rule, (a + b) ÷ (a − b) = (c + d) ÷ (c − d).