A real age problem!

Algebra Level 3

Red is 17 years younger than Sepia. If Sepia's age is written after Red's, the result is a 4-digit perfect square. The same situation recurs after 13 years. Find the age of Red if Sepia is older than 30.


The answer is 19.

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1 solution

Chew-Seong Cheong
Oct 25, 2018

Let the ages of Red and Sepia be r r and s s respectively, Then s = r + 17 s=r+17 and that

{ 100 r + s = 101 r + 17 = m 2 . . . ( 1 ) 100 ( r + 13 ) + s + 13 = 101 r + 1330 = n 2 . . . ( 2 ) where n > m are positive integers. \begin{cases} 100r + s = 101r + 17 =\color{#3D99F6} m^2 & ...(1) \\ 100(r+13) + s+13 = 101r + 1330 = \color{#3D99F6} n^2 & ...(2) \end {cases} \small \color{#3D99F6} \text{ where }n > m \text{ are positive integers.}

( 2 ) ( 1 ) : n 2 m 2 = 1313 ( n m ) ( n + m ) = 13 × 101 Note that 13 and 101 are primes. \begin{aligned} (2)-(1): \quad n^2-m^2 & = 1313 \\ (n-m)(n+m) & = \color{#3D99F6} 13\times 101 & \small \color{#3D99F6} \text{Note that 13 and 101 are primes.} \end{aligned}

Assuming { n m = 13 n + m = 101 m = 44 m 2 = 19 36 r = 19 \begin{cases} n - m = 13 \\ n+m = 101 \end{cases} \implies m = 44 \implies m^2 = {\color{#3D99F6}19}36 \implies r = \boxed{\color{#3D99F6}19} .

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