The average of four numbers is 20. If the average ofthe first two numbers is 15, then what is the average of the last two numbers?
let the four numbers be a,b,c,d
average of those 4 numbers is 20
$$\frac{\left(a+b+c+d\right)}{4}$$=20
a+b+c+d=80
also given that avg of first two numbers is 15
i.e; $$\frac{\left(a+b\right)}{2}$$=15
a+b=30
we need to find average of last two numbers
30+c+d=80
c+d=50
avg is $$\frac{\left(c+d\right)}{2}$$
50/2= 25
hence answer is option C
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