Question

What number must be added to each of the numbers 5,7,9,12 to get the numbers which are in proportion

Answer 1

Let x is added to all numbers,

$\frac{5 + x}{7 + x} = \frac{9 + x}{12 + x}$

$60 + 17x + {x}^{2} = 63 + 16x + {x}^{2}$

$x = 3$