Question

how to find percent of a number mentally

Answer 1

There are some percentages that are easy to calculate mentally. 50% means half, 25% means 1/4 and 75% means 3/4 .Finding 10% of a number is one of the most useful and also one of the easiest. To find 10% all we need to do is move/shift the decimal one place to the left. Thus 10% of 543 = 54.3, pretty simple! 1% is just as easy too. Shifting the decimal point to 2 digits left. So, 1% of 654 = 6.54 100% is the easiest by far. The number does not change. So, 100% of 765 is the number itself i.e. is 765.50% is just the number divided by 2. So, 50% of 444 = 222.How 5% works. It’s just half of 10%. Simply 2 steps in any order (1) Take 10% (2) Divide by 2. You can take 10% first or divide by 2 first, it does not matter. For example, 5% of 44 =? 10% of 44 is 4.4 and divide by 2 gives us 2.2. So, 5% of 44 = 2.2. Again, 5% of $24.80=? a) $24.80 -->> $2.48 (moving the decimal point one place to the left) and b) $2.48 / 2 = $1.24.15% is just 10%+5% and we already know how to solve these. So, 15% of 24 =? 10% of 24 is, well, take the decimal to one digit left to get 2.4 and 5% is just half of 10%. So we divide 2.4 by 2 to get 1.2. So, add them up 2.4+1.2= 3.6 which is 15% of 24.20% of 120=? 20% is nothing but 10%+10% (breaking up into friendly parts). So, 10% of 120 is 12 (shifting the decimal one place to the left) and double of that would be 24. Again, 20% of 132=? 10% would be 13.2 and 20% would be double of that that is 26.460% of 320=? 60%=50%+10%, so 50% of 320 would be 160 and 10% would be 32. So, our final answer would be 160+32 which is 192.75% is doing the same thing-breaking it up into friendly parts/numbers. 75% of 880=? You can break it up by 50%+25%. 50% of 880 gives us 440. Now we need to find 25% of 880. Now, 25% is just half of 50% and we already solved 50%, so just take half of that. 440 divided by 2 gives us 220. Now as we broke it up, we have to put it back together. So our final answer is 440+220 =660.95% of 2400 =? Break it up into friendly parts/numbers. So break it 2 parts- 100% and 5%. The 100% of 2400 does not change. So, 100% of 2400=2400 and 5% of 2400=? To find 5%, take 10% and then divide by 2. 10% of 2400 is 240 and half of 240 is 120, so 5% of 2400 is 120. Now as we broke it up, we have to put it back together. So, 100% - 5% =2400-120=2280.110% of 140=? 110%=100%+10%, 100% is the number itself which is 140 and 10% of 140 is 14 so 140+14=154.We can use same breaking up techniques to solve ANY percentage problem.

31% of 222=? Breaking up 31% by 10%+10%+10%+1%. Now, 10% of 222 just gives us 22.2. Because we have 3 sets of 10%, we need to multiply 22.2 by 3 to get 66.6 (10%+10%+10% is the same as 3x10%). Lastly we need to find 1% of 222 which gives us 2.22. So, our final answer is 66.6 +2.22 =68.8221% of 150=? 21%=10%+10%+1%. Now 10% of 150 would be 15. So, 20% would be double of 15 that is 30 and1% of 150 would be 1.5. So final answer would be 30+1.5 which is 31.517% of 300=? Break 17% into 10% + 5% + 2%. Now calculate; 10% of 300 is 30, 5% of 300 is half of 10% (i.e. 30/2= 15) and 2% (2x1% of 300 = 2x3 = 6). So it all add up to 51.Let’s try 97% of 195. 97% is probably broken down easiest into 100%-3%. 1% of 195 =1.95. Now multiply it by 3 to get 5.85. So our final answer is 195-5.85=189.1598% of 250=? 98% is 100%-2%. 100% of 250 is 250 and 1% would be 2.5 (decimal will shift 2 places towards the left). 2% would be 5. So our final answer would be 250-5 i.e. 245.167% of 195=? One way to break it up is 100%+50%+10%+5%+2%. We know 100% of 195 is 195. 50% is just half of 195 which is 97.5. 10% of 195 is just 19.5, 5% is 9.75, 2% of 195 is 3.9. And finally we just add them all up to get, 195 + 97.5 + 19.5+9.75+3.9 = 325.65There are many different ways to look at a problem. Say, 42% of 366=? 2 different ways to solve the same problem. We can do this by taking (a) 4x10% + 2x1% or (b) we can also do it a different way by taking 50% and subtracting 8% i.e. 50% - 1%x8. Both are same, both give us the same answer.  a) 4x10% + 2x1%. 10% is 36.6 and 1% is 3.66. Now, 4x36.6=146.4 and 2x3.66=7.32; adding them up is 153.72. b) 50% - 1%x8. 50% is half of 366 which is 183 and 8% is 3.66x8 = 29.28; subtracting the two gives us 183-29.28=153.72, the same answer as before.62% of 420 would be 50%+10%+2% which is 210 + 42 + 8.4 = 260.422% of 324 would be 10%+10%+2% i.e. 32.4×2 +3.24×2 = 64.8 + 6.48 = 71.2816% of 42 would be 10%+5%+1% i.e. 4.2 + 2.1 + 0.42   = 6.72.
a% of b = b% of a
It may not be immediately clear, but it’s true: a% of b = .01 × a × b, which is the same as b% of a (.01 × b × a).What’s 16% of 25? The same as 25% of 16: 4What’s 43% of 200? Same as 200% of 43: 86.