Shortcut Trick
For numbers a, b, c, d in proportion after adding x, use the formula:
x = (ad − bc) ÷ (a + d − b − c)
⇒ x = (12 × 45 − 28 × 21) ÷ (12 + 45 − 28 − 21)
⇒ x = (540 − 588) ÷ (57 − 49) = −48 ÷ 8 = −6 (Taking magnitude, x = 6)
Mean Proportional = √((6 + 3) × (4 × 6 + 1)) = √(9 × 25) = 3 × 5 = 15.
∴ The correct answer is 15.
Alternate Method
Given: Numbers are 12, 28, 21, 45. x is added to each.
Formula Used: If a, b, c, d are in proportion, then a/b = c/d. Mean proportional of p and q is √(p × q).
⇒ (12 + x) ÷ (28 + x) = (21 + x) ÷ (45 + x)
⇒ (12 + x)(45 + x) = (21 + x)(28 + x)
⇒ 540 + 12x + 45x + x2 = 588 + 21x + 28x + x2
⇒ 540 + 57x = 588 + 49x
⇒ 57x − 49x = 588 − 540
⇒ 8x = 48 ⇒ x = 6
⇒ First term = x + 3 = 6 + 3 = 9
⇒ Second term = 4x + 1 = 4 × 6 + 1 = 25
⇒ Mean Proportional = √(9 × 25) = √225 = 15
∴ The correct answer is 15.
Additional Information
Mean Proportional
The mean proportional between two numbers a and b is defined as √(ab).
Third Proportional
If a, b, c are in continued proportion, then c = b2/a is the third proportional.
Fourth Proportional
If a, b, c, d are in proportion, then d = (b × c) / a is the fourth proportional.