Find the fourth proportional to 1.5, 4.5 and 3.5

Let fourth proportional to 1.5, 4.5 and 3.5 be x

$\Rightarrow 1.5 : 4.5 = 3.5 : x \\[1em] \Rightarrow \dfrac{1.5}{4.5} = \dfrac{3.5}{x} \\[1em] \Rightarrow x = \dfrac{3.5 \times 4.5}{1.5} \\[1em] \Rightarrow x = 3.5 \times 3 \\[1em] \Rightarrow x = 10.5$

Hence, fourth proportional to 1.5, 4.5 and 3.5 is 10.5