There are 13 factor pairs of 10,000: 10000 = 1 x 10000 10000 = 2 x 5000 10000 = 4 x 2500 10000 = 5 x 2000 10000 = 8 x 1250 10000 = 10 x 1000 10000 = 16 x 625 10000 = 20 x 500 10000 = 25 x 400 10000 = 40 x 250 10000 = 50 x 200 10000 = 80 x 125 10000 = 100 x 100

the trick to this is to realize the pattern: 10000=0+10000 10000=1+9999 10000=2+9998 ... 10000=4999+5001 so there are 5000 (10000s) and the unmatched 5000 in the middle = 50,005,000