Question

Find the fourth proportional to
$42,12,7$

Answer 1

$42,12,7$
Let the fourth proportional be $x$
Then $42,12,7,x$ are in proportion
Using the cross product rule
Product of extremes = Product of means
$42\times x=12\times 7$
$x=\frac{12\times 7}{42}$
$x=\frac{84}{42}$
$x=2$