501
First, add the digits other than the units, 70 + 420 = 490. Remember that for a moment. Now, add the one's digits, 5 + 6 = 11. Now, add the 11 to the 490 to get 501. ---- Alternately, express 426 = 400 + 25 + 1. Then 426 + 75 = 400 + (75 + 25) + 1 = 400 + 100 + 1 = 500 + 1 = 501