Question

What is the missing number in the given series $2:5::7:?$

Answer 1

Determine the missing number in the given series $2:5::7:?$.

Let the missing number be $a$.

Calculate the missing number:

$\begin{array}{rcl}\frac{2}{5}& =& \frac{7}{a}\\ & \Rightarrow & 2a=7\times 5\\ & \Rightarrow & 2a=35\\ & \Rightarrow & a=\frac{35}{2}\\ & \Rightarrow & a=17.5\end{array}$

Hence, the missing number in the given series is $17.5$.