For example, if you're looking for a factor pair that includes 13:
Start with 12:
420 ÷ 12 = 35
35 - 12 = 23 (not 13, so keep going)
Try 14:
420 ÷ 14 = 30
30 - 14 = 16 (still not 13)
Finally, test 15:
420 ÷ 15 = 28
28 - 15 = 13 (found the correct pair: 15 × 28 = 420)
This method helps you quickly zero in on the right factors without listing them all manually.