Multiply a number by 4 is doubling a number twice, i.e. you double the number 2 times (as 4 can be written as 2 x 2).

For example:

- 45 x 4 = double of (double of 45)

Double of 45 is 90, and double of 90 is 180.

So, 45 x 4 = 90

- 86 x 4 = double of (double of 86)

Double of 86 is 172, and double of 172 is 344.

So, 86 x 4 = 344

Multiply a number by 8 is doubling a number thrice, i.e. you double the number 3 times (as 8 can be written as 2 x 2 x 2)

For example:

- 45 x 8 = double of (double of (double of 45))

Double of 45 is 90, and double of 90 is 180, and double of 180 is 360.

So, 45 x 8 = 360

- 86 x 8 = double of (double of (double of 86))

Double of 86 is 172, and double of 172 is 344, and double of 344 is 688.

So, 86 x 8 = 688

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