We need to calculate 21% of 578. A good approximation of a percentage change can be done using the 10-1 approach. In the 10-1 approach, one starts by calculating the rounded off values representing 10% and 1% of the number. Now 10% of 578 is 58 (rounded off) and 1% is 6 (rounded off).
Now we can calculate any percentage of this number by using 10% and 1%
To calculate 21% of this number, we start by getting 20%, which is 10%*2 = (58 X 2) = 116. We now add 1% i.e. 6 in this and get the answer as 122.
To calculate 19%, we subtract 1% from 20% and get the answer as 116 - 6 = 110.
Now let's say a problem requires us to calculate 52% of 281. Its 10% is 28 and 1% is 3, both being rounded up values. First we find 50% of this number i.e. 140 (half of the number) and then 2% of this will be added in this i.e. 6 to get the answer as 146.
Knowledge of percentage change is also important for cracking data interpretation questions. The formula that we use for percentage change from P to Q is: 100 * (Q - P)/P. In the pressure of an exam, especially when we are working with larger numbers, we take more time when we write. Let's try to do this mentally.
We have to find the percentage change from 271 to 353. Here the difference between the two numbers are 82 and 10% of the base value i.e. 271 is 27. How many 27s can fit into 82? Three times of 27 is 81. Hence it is approximately 30%.
Let us take another example, say find percentage change from 911 to 938. The difference between the two is 27. In this case 10% is 91 and 1% is 9. Multiplying 9 by 3 we get 27. Hence percentage change is 3%.To conclude, the 10-1 approach can be used to be able to calculate percentage change mentally, thereby saving precious time in an exam!
Team Bulls Eye