The trick. Every number equals its digit-sum, reduced to one digit (mod 9): 47 → 4+7=11 → 2. For a real product the digit-roots must line up:
47 × 53 = 2491? root(47)=2, root(53)=8, 2×8=16→7. root(2491)=7. Match → could be right.
47 × 53 = 2490? expected root 7, but root(2490)=6. Mismatch → definitely wrong.
If the roots don't match, the answer is wrong — guaranteed. Tap ✓ Looks right or ✗ Wrong; the working is shown after each. 90 seconds.