When we say a number is +23 then we mean it is 23 more than 0. similarly, when we say a number is equal to -10 that means that number is 10 less than 0. So the standard we have in our mind is 0. When we change the standard to an another value like 5 then +23 will become 23-5=+18 and -10 will become -10-5=-15. We can call 18 and -15 as deviations with respect to 5. The same concept can be used to calculate average faster by changing the standard to a different value. The concept is called the concept of the assumed mean.
For example,we have to find average of 140,134,136,132,143
We can take any value as assumed mean and find deviation with respect to that value. If we take 130 as assumed mean then we can write these values as 10, 4, 6, 2 ,13. The averages of these 5 numbers is 35/5=7 so the overall average will be 130+7=137.
To learn this concept in detail, understand the concept from this picture.
Now, we will practice a few exams and understand how to solve the questions using it.