# While calculating the average of eight 2-digit numbers, a student mist

| While calculating the average of eight 2-digit numbers, a student mistakenly interchanged the digits of a number as a result the average becomes 9 less than the original average. What is the difference of two digit of that number?

A. 6

B. 7

C. 4

D. 8

### Right Answer is: D

#### SOLUTION

Let the unit and tenth digit of the original number is y and x respectively. Then, the original number is (10x+y) and interchanged digit number is (10y+x). Let the sum of remaining seven numbers is S and the original average is A. Then,

S+(10x+y)=8A……(i)

and

S+(10y+x)=8(A-9)

S+(10y+x)=8A-72……(ii)

Subtracting second from first we get

10x+y-10y-x=72

9x-9y=72

x-y=8

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While calculating the average of eight 2-digit numbers, a student mist