Percentage Arithmetic

[maverik_]

Hello:

How can i solve this quickly in my head no pen or paper:

20% of 6945

 

[wjm11]

Hello:

How can i solve this quickly in my head no pen or paper:

20% of 6945

Another approach is find 10% first by simply moving the decimal point one place to the left:

10% of 6945 = 694.5

To get 20%, just double that number: 2(694.5) = ?

 

[HallsofIvy]

Another way: \(\displaystyle 20\%= \frac{20}{100}= \frac{2}{10}\) so start by doubling the number: 2(6945)= 13890 (you can double in your head, can't you?) Then divide by 10 by moving the decimal point over:1389.0.

 

[Deleted member 4993]

Another way: \(\displaystyle 20\%= \frac{20}{100}= \frac{2}{10}\) so start by doubling the number: 2(6945)= 13890 (you can double in your head, can't you?) Then divide by 10 by moving the decimal point over:1389.0.

or first approximate 6940 ≈ 7000 ( approximating in the very begining is a bit dangerous!)

then 7000 * 2 = 14000

then 14000/10 = 1400

 

[JeffM]

Hello:

How can i solve this quickly in my head no pen or paper:

20% of 6945

Usually in mental arithmetic you are not supposed to find an exact answer but an approximate answer.

\(\displaystyle 20\%\ of\ 6945 \approx 0.2 * 7000 = 2 * 700 = 1400.\)

Mental arithmetic is (at least usually) about making the computations as easy as possible at the expense of losing an exact answer.

Notice that the exact answer is 1389, which differs from the approximate answer by less than 1%.

You need to find out whether this test demands exact answers or approximate answers.

 

[JeffM]

OMG!!
.2 * 7000 = 2 * 700
2 * 700 = 1400
To the corner, and no dessert!
:shock:

Well, Denis, at least I know that you will be in the corner with me. I don't know what you have done, but whatever it was was bad enough. As for me I am just a reprobate.