The Number Puzzle

Algebra Level 2

There are two positive numbers with the difference of 3 between them and the difference of their squares is 51. What is the smallest of the two numbers?


The answer is 7.

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2 solutions

Nashita Rahman
Feb 11, 2017

Let the two positive numbers be x and y and x>y.

x - y = 3 ....(1)

x^2 - y^2 = 51

We know , x^2 - y^2 = (x + y)(x - y)

Or , x + y = 51/3 = 17 ....(2)

Adding eqn (1) & (2),

2x = 20 or x = 10

y = x-3 = 7

Therefore the smallest of the two is 7.

You should specify that the numbers are positive. Otherwise you could have -10 and -7. In which case the solution would be -10.

Denton Young - 4 years, 1 month ago

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Oh yes , thank you for pointing out the mistake . I did edit the problem and it's fine now .

Nashita Rahman - 4 years ago
Mohammad Khaza
Sep 27, 2017

suppose, the two positive numbers are x & y and \text{suppose, the two positive numbers are x \& y and} x > y x>y

now, \text{now,} x y = 3 x-y=3 .......................[1]

and \text{and} x 2 y 2 = 51 x^2-y^2=51 .................[2]

now, dividing [2] by [1] we get, \text{now, dividing [2] by [1] we get,}

x 2 y 2 x y \frac{x^2-y^2}{x-y} = 51 3 =\frac{51}{3}

or, \text{or,} x + y x+y = 51 3 =\frac{51}{3} .........................[3]

now, doing [1]+[2] we get , \text{now, doing [1]+[2] we get ,}

x y + x + y = 3 + x-y+x+y=3+ 51 3 \frac{51}{3}

or, \text{or,} 2 x = 20 2x=20

or, \text{or,} x = 10 x=10

or, \text{or,} y = 10 3 = 7 y=10-3=7 .........[answer]

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