If the third proportional of 28 and 70 be x, then what is the value of x?

Correct option is D

​$$\textbf{Given:} \\\text{First number} = 28 \\\text{Second number} = 70 \\\text{Third proportional} = x\\\textbf{Formula Used:} \\\text{If } a : b = b : c,\ \text{then } c = \dfrac{b^2}{a}\\\textbf{Solution:} \\28 : 70 = 70 : x \\\Rightarrow \dfrac{70}{28} = \dfrac{x}{70}\\\text{Cross multiplying:} \\28x = 70 \times 70 \\28x = 4900\\x = \dfrac{4900}{28} \\x = 175\\\\\textbf{Therefore, the value of } x = \boxed{175}$$​​