How do you easily multiply 11 numbers mentally? [See question first please]I got a problem asking me to find the tens' digit of the product of 4*5*6*7*8*9*10*11*12*13*14. It was 5 minute multiple...

How do you easily multiply 11 numbers mentally? [See question first please]

I got a problem asking me to find the tens' digit of the product of 4*5*6*7*8*9*10*11*12*13*14. It was 5 minute multiple choice test our teachers uses as a bonus test. I picked 0 and now that I checked it I know its 0. But how do  I find the 10s place mentally. Easiest method get the best answer! Tje first answers might also get it!

Asked on by ibleed

1 Answer |Add Yours

thilina-g | College Teacher | (Level 1) Educator

Posted on

`4xx5xx6xx7xx8xx9xx10xx11xx12xx13xx14`

I can rewrite this as below.

`2xx2xx5xx6xx7xx8xx9xx10xx11xx12xx13xx14`

`2xx6xx7xx8xx9xx11xx12xx13xx14xx2xx5xx10`

`2xx6xx7xx8xx9xx11xx12xx13xx14xx10xx10`

This gives,

`2xx6xx7xx8xx9xx11xx12xx13xx14xx100`

Now whatever the answer of` 2xx6xx7xx8xx9xx11xx12xx13xx14` it will be multiplied by 100. That means two zeros will be added to end. Therefore in the final answer ones' is also 0 and tens' is also zero.

So your answer is correct.

Now your above question how to multiply easily by 11. You can use the following.

`33 xx 11 = 3(3+3)3 = 363`

You add the two numbers and write the two numbers on either side of that addition.

`66 xx 11 = 6(6+6)6 = 6(12)6 = (6+1)26=726`

When you have the addition more than 10 you can simply transfer tens' digit to left side.

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