The value of v/c will always be chosen to be the ratio of one of the legs of a triangle to its longest side. The square-root in the formula will then be:
Two events on the ship are separated by three years of ship-time. How many earth-years separate the events?
The problem statement tells you that v/c = 4/5. Look at the 3-4-5 triangle. The leg not used yet is 3, so the square root in the formula must be equal to 3/5. The problem statement tells you that T = 3 years elapse on the ship. The earth-time is then 3 years divided by 3/5. That is the same as 3 years multiplied by 5/3 or 5 years.