6398+4136 = (6400+4136)-2= (6400+4100)-2+36 = 10500-2+36 = 10534
9296-7258 = (9300-7258)-4= (9300 - 7300) + 42 - 4 = 2000 + 42 -4 = 2038
3683 - 1398 = (3683 - 1400) + 2 = (3600 - 1400) +83 + 2 = 2200 + 85 = 2285
17591 - 9295 = (17590 - 9245) +1 - 50 = 8345 - 49=8345 - 45 - 4 = 8300 - 4 = 8296