The ratio of ages of two persons $A$ and $B$ is $3:4$ and that of ages $B$ and $C$ is $4:5$. If the average of all three is $40$ years, what is $B$’s age?

$A:B:C$ is be $3:4:5$ and $A+B+C =40*3= 120$

$B’s$ age = $\frac{4}{12}*120 =40$

Option C) is correct