Question

what number should be added to each of the numbers $12,22,42$ and $72$ such that they are proportional

Answer 1

Given, $12,22,42,72$

Let the number to be added be $x$

$12+x:22+x::42+x:72+x$

$\Rightarrow (12+x)(72+x)=(22+x)(42+x)$

$x^2+84x+864=x^2+64x+924$

$20x=60$

$\therefore x=3$