126 x 20% is the same as 126 x .20 = 25.2, so you are getting 25 dollars and 60 cents off the price....
126$ - 25.20$ = 100.80$
now you know how to do it!
also to do something like this in your head you can think about it this way:
20% is really 10% + 10% (since 10 + 10 = 20).
10% of a number is really easy to do, you just move the decimal point over to the left 1 space.
So for example:
10% of 50.00$....we move the decinal point to the left 1 space, 5.000 which is really just 5.00$ so 10 percent of 50.00$ is 5.00$
so 20% of 50$ is 5$+5$=10$
or even easier 20% of 10.00$ is 10% of 10$ plus(+) 10% of 10$
10% of 10.00$ , we move the decimal point 1.000$ or simply 1.00$
so 1.00$ + 1.00$ = 2.00$, so 20% of 10.00$ is 2.00$
so give yours a try!
20% of 126.00$ is really 10% of 126.00$ plus(+) 10% of 126.00$
10% of 126.00$ ---> 12.600$ or simply 12.60$
So, 12.60 + 12.60...well lets just round off to 12.50 cause thats easier to do in your head, thats 12.50 +12.50 thats 25.00
so its roughly 25.00$ off. so thats about 101$!
Simple isn't it? dont let things overwhelm you, break them down into smaller parts.
The easiest way to calculate it while you're out shopping with no calculator is to take 10% of any dollar amount. Say if the price is $200, 10% of that is $20, If the price is $5, 10% of that is 50 cent...so basically it's about moving decimal in 2 times. And then if you need to figure 20%, take that number you figured from 10% and multiply it by 2, if it's 90%, multiply it by 9. If it's 75%, multiply it by 7.5.
參考: Part time waitressing experience. Word of advice from a waitress on how to figure out tip percentage in your mind.