# 5 cool math tricks ft. Technicality | Summary and Q&A

652.3K views
November 3, 2016
by
Physics Girl
5 cool math tricks ft. Technicality

## TL;DR

Learn simple math tricks including calculating tips, multiplying by 11, multiplying by 9, finding square roots, and using the Chinese remainder theorem for encryption.

## Install to Summarize YouTube Videos and Get Transcripts

### Q: How can I quickly calculate tips without using a calculator?

To calculate tips, multiply the total by 0.2 or move the decimal place over one and multiply by 2. For example, if the bill is \$30, the tip would be \$6 (30 x 0.2) or \$6 (30.0 x 2).

### Q: What is the trick for multiplying numbers by 11?

To multiply numbers by 11, separate the two digits of the number being multiplied and add them to get the answer. For example, 43 times 11 would be 473 (4 + 3 = 7, with the result between them).

### Q: How does the trick for multiplying by 9 using fingers work?

When multiplying by 9, subtract 1 from each 10s finger digit, and set aside the subtracted ones. Then, convert the last 10s finger into 10 1s fingers. The number of remaining fingers represents the result. For example, 4 times 9 would be 36 (minus four 1s).

### Q: How can I find the square root of a number without a calculator?

Use a geometric method where you form a right triangle with side lengths that match the square root of the number you're trying to find. Then, calculate the hypotenuse of this triangle to estimate the square root.

## Summary & Key Takeaways

• The video showcases various math tricks, like a quicker method to calculate tips and multiplying by 11 and 9.

• It explains the concept behind multiplying by 9 using your fingers and why it works.

• The video also demonstrates a geometric method to find square roots and how the Chinese remainder theorem is used in encryption.