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4.Average
hard
A student finds the average of $10,2$ digit numbers. If the digits of one of the number is interchanged, the average increases by $3.6 .$ The difference between the digits of the $2 -$digit numbers is
A
$4$
B
$3$
C
$2$
D
$5$
Solution
Let Average $_{10}=y$
$\operatorname{Sum}_{10}=10 \times y$
It digits of $1$ number is interchanged, new average $=y+3.6$
$Sum _{10}$ after interchanging digits $=10 y+36$
$ab \rightarrow 2$ digit no\% whose digits have been interchanged
$9(b-a)=36$
$b-a=4$
Standard 13
Quantitative Aptitude