Time varying resistance in circuits

[Tanishq Nandan]

Homework Statement

A battery of 10 volts is connected to a resistance of 20 ohms through a variable resistance R.The amount of charge which has passed through the circuit in 4 minutes,if the variable resistance R is increased at the rate of 5ohms per minute is:
A)120 C
B)120ln2 C
C)120/ln2 C
D)60/ln2 C

Homework Equations

V=IR(Ohms law)
I=dq/dt
Integration of 1/(ax+b)=[ln(ax+b)]/a
Unitary method:
Rate of increase of resistance=5 ohms per minute or...1/12 ohms per second

The Attempt at a Solution

But,my answer isn't in the options...answer's B

 

[WrongMan]

Your problem is the substitution of bounds into the integral
What happened to the "lower bound"?