Ask yourself: by what number (a fraction) must we multiply 4 to obtain 13/3? Try (13)/(4*3).
Check: Does 13
 ------- * 4 equal 13/3? Yes.
 4(3)
Check again. Does 13 13
 --------- * ------- = 14/3 ? No
 4(3) 3
Try something different. Convert 4 to 3(4)
 -----
 3
Then we have 12/3, 13/3, 14/3, 15/3, 16/3 (different numerators, same 
 denominator)
Continuing, 17/3, 18/3, 19/3, and so on.