The fast method
⚡
Digit sum = 5. Expanded form: 200 + 50 + 3 = 253.
How it works
Add its digits and insert the sum between them, carrying when needed. 11(10a+b) = 100a + 10(a+b) + b.
Why it works
11(10a+b) = 100a + 10(a+b) + b.
Use it when
Use this digit method for positive two-digit integers.
Watch out for
If the digit sum exceeds 9, carry 1 into the hundreds place.
Verified examples
Example 1
23 × 11
Digit sum = 5. Expanded form: 200 + 50 + 3 = 253.
253Example 2
54 × 11
Digit sum = 9. Expanded form: 500 + 90 + 4 = 594.
594Example 3
78 × 11
Digit sum = 15. Expanded form: 700 + 150 + 8 = 858.
858