f = (f1 f2) / (f1 + f2)
f(f1 + f2) = f1 f2
f f 1 + f f 2 = f1 f 2
f1 f2 - f f2 = f f1
f2 (f1 - f) = f f1
f2 = (f f1) / (f1 - f) <==== solution
anything multiplied by one is itself, anything multiplied by zero is zero
For this, we need to find the lowest common multiple of 12 and 15....
common multiples of 12 : 12,24,36,48,60
common multiples of 15 : 15,30,45,60
LCM = 60
the caller that will be the first to win both is the 60th caller
For this case we have the following equation:

We must clear the variable "x":
We multiply by 2 on both sides of the equation:

We divide by h on both sides of the equation:

We subtract "a" on both sides of the equation:
Answer:
Mmmm c i’m thinking it’s that