Practice: compute 10,000 − 6,789 mentally.

3,211 — use complements: 9,999 − 6,789 = 3,210, then add 1.

A fast route for subtracting from a power of 10:

9,999 − 6,789 = 3,210 because each digit complements to 9. Since the real top number is 10,000, add 1:

3,210 + 1 = 3,211.

This is just counting up in disguise: 6,789 → 9,999 is 3,210, then one more step reaches 10,000.