The Speedy 11x Multiplier!

Ever wished you could multiply ANY two-digit number by 11 in a flash, without a calculator? Get ready for a mind-blowing shortcut!

Subject: mental-calculation • Classes: 6–12 • Difficulty: basic

The Trick

Let's multiply any two-digit number $AB$ by 11. 1. Take the two digits of the number, say $A$ (tens place) and $B$ (units place). 2. Write $A$ as the first digit and $B$ as the last digit, leaving a space in between: $A\_B$. 3. Add the two digits together: $S = A + B$. 4. Place the sum $S$ in the middle space. 5. If $S$ is a single digit (0-9), that's your answer! If $S$ is a two-digit number (10 or more), write down the units digit of $S$ in the middle and 'carry over' the tens digit of $S$ to the first digit $A$. Add the carried digit to $A$. WHY IT WORKS: Think of it algebraically! $11 \times (10A + B) = 110A + 11B = 100A + 10A + 10B + B = 100A + 10(A+B) + B$. This literally puts $A$ in the hundreds, $(A+B)$ in the tens, and $B$ in the units place, just like our trick!

Study More with GyanAI

GyanAI is a free AI tutor for CBSE students. Ask any question for an instant step-by-step answer. Try GyanAI free.