What resistance is needed in this rc circuit if the flash is to charge to 90% of its full charge in 22 s ?

Missing part in the text of the problem:
"a flash unit for a camera has a capacitance of1200μF."

Solution:
In a RC circuit, the charge of the capacitor at time t follows the relationship:

where  is the full charge, and  is the time constant of the circuit. 

We can isolate  from the previous equation:




We can now using the data of the problem. We know that after a time t=22.0s, the capacitor is at 90% of tis charge, therefore . So we find


And from this value we can find the value of the resistance R, since we know that . Given , we have

The answer is b hope this