How can we efficiently multiply by multiples of 10?

just add a zero

Step-by-step explanation:

because if you do all the work it's just a waste of time when you could get the same answer by just adding a zero.

$3720

step-by-step explanation:

taxable income = $31000

as per the 2018-2019 us tax bracket, the tax rate is 12% for an individual whose taxable income is between $9,526 to $38,700.

so, the tax to be paid by him = 12% of 31000

= 0.12 * 31000     [12% converted to decimal as 0.12]

= $3720