Divisibility by 5

10 is divisible by 5. Every number with last digit 0 is also divisible by 5, because we can present it as sum of tens.

For example: 40 is divisible by 5 , because
40 = 10 + 10 + 10 + 10

Multi-digit number, which doest not have last digit 0, we can present as sum of number with last digit 0 and ones.

For example: 425 = 420 + 5
428 = 420 +8

425 is divisible by 5 because the two addends(420 and 5) are divisible by 5
428 is not divisible by 5 because one of the addends(8) is not divisible by 5

So, if a multiciphered number can be divided by 5 depends only on the digit of the ones; If this digit is 5 or 0, the number is divisible by 5

Divisibility by 5 - examples:
Numbers 105, 275, 315, 420, 945, 760 can be divided by 5
Numbers 151, 246, 879, 1404 can not be divided by 5

A quick way to divide numbers by 5

Suppose you would like to divide 342 by 5
342:5 = 68.4

First step:
Multiply the number by 2.
342 x 2 = 684

Second step:
Move the decimal point by one place to the left and the result is 68.4

Example 2:

Divide 415 : 5 = ?
First step: 415 x 2 = 830
Second step: 83.0 or result is 83.

A quick way to multiply by 5

Let's multiply quickly without a calculator 62 x 5 = ?

First step:
Divide 62 by 2 - 62:2 = 31

Second step:
Multiply the result by 10.
31 x 10 = 310

So 62 x 5 = 310

Square two-digit numbers ending in 5

Example: 35² = ?

First step:
Multiply the first digit by the next number: 3 x 4 = 12

Second step:
Append 25 at the end
and the result is 1225.