Question

The square of a number when decreased by 15 is twice the original number. Then the number is:

• A. 8 or 7
• B. 9 or 6
• C. 2 or -7
• D. 5 or -3

Answer 1

The correct option is D

5 or -3

Let the number be x

Given, $x^2-15=2x$

$\Rightarrow$ $x^2-2x-15=0$

$\Rightarrow$ $x^2-5x+3x-15=0$

$\Rightarrow$ $x(x-5)+3(x-5)=0$

$(x-5)(x+3)=0$

$\Rightarrow$ $x=5andx=-3$