If a car covers 2/5^th of the total distance with $v_1$ speed and 3/5^th with $v_2$ then average speed is

$\text{Average}\text{speed=}\frac{\text{Total}\text{distance}\text{travelled}}{\text{Total}\text{time}\text{taken}}\text{=}\frac{\text{x}}{{\text{t}}_{\text{1}}{\text{+t}}_{\text{2}}}$

$=\frac{x}{\frac{\left(2/5\right)x}{v_1}+\frac{\left(3/5\right)x}{v_2}}=\frac{5v_1{v}_2}{2v_2+3v_1}$