Answer:
f the mean of this set is equal to 20, we can write down the below equation,
20 = (x1 + x2 +x3 + .... + x10)/10
x1 + x2 + x3 + ... x10 = 200
Then we can also write an equation for the mean of the given numbers as below,
Mean = [(x1+4) + (x2+8) + (x3+12) + .... + (x10+40)]/10
= (x1 + x2 + x3 + ... + x10 + 4 + 8 + 12 + ... + 40)/10
Then we can use above equation (1) to replace x1 + x2 + x3 + ... + x10 by 200
Mean = (200 + 4 + 8 +12 + 16 + 20 + 24 + 28 + 32 + 36 + 40)/10
= 420/10
= 42
If you remember Arithmetic Progressions you can simply add together the above number set.
If you closely look above, you can find that there is an Arithmetic Progression : 4, 8, 12, ... , 40
Here we want the addition of 10 terms. So we can use,
Sn = n/2(a+l)
S10 = 10/2(4+40)
= 220
Then you can easily get the answer,
Mean = (200 + 220)/10
= 42