13•8=104. 13+8=21. To be honest I just used process of elimination. You can do this by listing the factors of 104, which are 1 & 104, 2 &52, 4 & 26, and 8 & 13. Then you add the pairs. 104+1=105, 105 does not equal 21. 52+2=54, 54 does not equal 21. 26+4=30, 30 does not equal 21. This means the answer has to be 13 & 8 because 13+8=21, and 21 is equal to 21.