Speed problem with current(Resolved)

Hi I'm trying to solve this problem, but I can'T get the solution exempt by try and error. What am I doing wrong?

Problem:

A ship travels along a river from A to B and back. Time required for both ways is 11,5 hrs. The river has a current of 2,5 Knots(Nm/hr). The distance from A to B is 91 Nm.
What's the speed of the ship (speed thru water!)

I have:
T1= Time to travel from A to B (with the current)
T2= Time to travel from B to A (against the current)
V= Speed of Ship (Speed thru the water)
T2=11,5 hrs -T1
The equation is:
2*91= T1*(V+2,5) + (11,5-T1)*(V-2,5)

I can convert this one back and for, but T1 and V will allways be in there. How would I get the correct value for V?

Note 15,8 is not the coorect one, this one would assume no current and T1=T2!
By try and error I got 16,2 Kn

T₁ + T₂ = T = 11.5 h
V_r = 2.5 Nm/h
d(A-B) = 91 Nm

Time for A to B travel: T₁ = d / (V + V_r)
Time for B to A travel: T₂ = d / (V - V_r)

Add these 2 up:

T = d[1/(V + V_r) + 1/(V - V_r)]

The rest is easy and you come up with a second degree equation:

V² - (2d/T)V - V_r² = 0

Solving for V you get 16.212 Nm/h

Thank's, so the problem was not to start with TotalTime.