It's the time in the watch.
Such as 10 A.M.+ 4 hours = 2 P.M.
How many ways can you find to make the number 10, by adding or subtracting prime numbers?
The same numbers but in a different order count as the same answer. (Prime numbers are only divisible by themselves and 1 so they are 2, 3, 5, 7, 11, 13, ...)