Question

In the given question, select the related number from the given alternatives.
$20$ $:$ $30$ $:$ $:$ $?$ $:$ $72$

• A. $56$
• B. $59$
• C. $68$
• D. $61$

Answer 1

Answer: (A)

$56$

All the given numbers can be expressed in terms of an integer with following relation:
$x:(x^2+x)$ for first number
and
$(x+2):\left(\right(x+2{)}^2-(x+2))$ for second number
For $x=4$
$20=(4^2+4)$
And
$\left(\right(4+2{)}^2-(4+2)=(6^2-6)$
For $x=9$
$72=(9^2-9)=(7+2{)}^2-(7+2)$

Therefore, $x=7$ and for $x=7$

$(7^2+7)=49+7=56$

Hence, the missing number is $56$.