11-1

Reason: 11 minus 1 equals 10. In any base b, 11_b − 1_b = 10_b as well (since (b+1) − 1 = b).

Want a few more quick subtraction problems for practice? Here are three:

• 23 − 7 = 16
• 100 − 58 = 42
• 501 − 199 = 302