reasons problem with 3 variables

Question

If a/3 = b/5 = c/2and a + b + c = 40, so 3a + b – 2c

It is a problem of reasons and proportionality, which I still can not solve, can someone explain to me how to arrive at the correct result? w why

Answer 1

$a/3$ = $c/2$

$a= 3c/2$

$b/5$=$c/2$

$b=5c/2$

If $a+b+c=40$ then $(3c/2)+ (5c/2) + c = 40$

$c=8$

Putting this on the other 2 equations

$b=20$

$a=12$

$3a+b-2c = 3\cdot 12 + 20 - 2\cdot 8 = 40$