Hello from MrBillDoesMath!
Answer: 15
Discussion.
Call the number referred to "n". Then the problem statement becomes:
4 + (3/5) * n = 13
Multiply both sides of the equation by 5 :
5 ( 4 + (3/5)*n) = 13 * 5 or
5*4 + 5 * (3/5)**n = 13*5
Cancelling the boldfaced 5's gives:
5*4 + 3*n = 13 *5 or
20 + 3*n = 65
Subtracting 20 from both sides gives:
3*n = 65-20 - 45
Dividing by 3 gives n = 15
Check: 4 + (3/5)*15 = 4 + 9 = 13! Bingo
Regards, MrB