What is 87 percent of 420?

Answer 1

In mathematics, estimation, is the ability to quickly deduce an approximate arithmetical solution.

When can use estimation to solve your problem.

Let's begin by asking ourselves what 10% of 420 is. If we look at percentages as being a simple "part of a whole" we understand that a whole is 100% and 10% is one of the ten that composes it.

Now a cheap trick of the trade for determining 10% of any number is to simply move the decimal place one place to the left.

Example: 10% of 1 would be .1 (1 ÷ 10 = .1). 10% of 963654.23 would be 96365.423 (or in money $96365.42) (96365.23 ÷ 10).

So with ours being 420, 10% would be 42.0.

Now we can estimate a little further. 42 x 8. 40 x 8 would be 320. 320 + 16 (the remaining 2) = 336.

We now have figured out what 80% of 420. It's 336. Now we can figure out very quickly that 5% of 42 is the same as 42 ÷ 2 = 21. 336 + 21 = 357. We're only 2% off so we can assume that 4.2 = 1% (since 5% = 21, 21 ÷ 5 = 4.2) so add on roughly 8.4. 357 + 8.4 = 365.4.

Is we did it on a calculator it would calculate to: 365.4, the exact sane as the total. Estimation can be use quickly, If one takes the time to work with numbers in their head.