Question

The number that must be added to each of the numbers $8,21,13$ and $31$ to make the ratio of first two numbers equal to the ratio of last two numbers is

• A. $5$
• B. $7$
• C. $9$
• D. $3$

Answer 1

Answer: (A)

$5$

Let $a=8,b=21,c=13$ and $d=31$

$\therefore$ required Number $=\frac{bc-ad}{(a+d)-(b+c)}$

$=\frac{21\times 13-8\times 31}{(8+31)-(21+13)}$

$=\frac{273-248}{39-34}$

$=\frac{25}{5}=5$