Easy math, but I'm dumb

Is it x/15 - x/20 = 1?

cause it fills at 1/15th per minute and drains at 1/20th per minute to change 1minute to t-minutes, set equation = one completed job?

I'd love to see your image, because I'm having difficulty understanding your problem. However, since you mention "drain" and "rate", I'm going to assume that this problem has something to do with filling and draining a tank. Your conditions state that this tank fills at 1/15th per minute and drains at 1/20th per minute. I'm going to assume that this means that it's being filled at 1/15th of its volume per minute and being drained at 1/20th of its volume per minute. Therefore, you have:

dx/dt = x/15 - x/20

dx/dt = x/60

dx/x = dt/60

∫dx/x = 1/60*∫dt

ln(x) = t/60 + C1

x = e^(t/60 + C1)

x = C2*e^(t/60), where C2 = e^C1

At time t=0, x = C2, so C2 = x(0). Therefore:

x = x(0)*e^(t/60)

Since this equation returns the volume at any time, plugging in 1 minute will reveal that you have e^(1/60) times what you had at the beginning of the filling/draining session.

I have no idea how Shidoshi produced 60 minutes as the answer, because that would mean that the problem states that x(60)=x(0)*e, which seems like a rather unlikely choice as an initial value for this problem. *shrugs*