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Leela and Madan pooled their music $CD's$ and sold them. They got as many rupees for each $CD$ as the total number of $CD's$ they sold. They share the money as follows: Leela first takes $10$ rupees, then Madan takes $10$ rupees and they continue taking $10$ rupees alternately till Madan is left out with less than $10$ rupees to take. Find the amount that is left out for Madan at the end, with justification.
Solution
(d)
Let the total number of CD's sold by the Leela and Madan together $=x$ Total money obtained by them
$=(x \times x)=x^2$
They divided $x^2$ in such that, $x^2=10$ (an odd number) $+$ a number less than $10$
$\Rightarrow \quad x=10 q+r \quad[\because 0 \leq r < 10]$
$\Rightarrow \quad x^2=(10 q+r)^2$
$\Rightarrow \quad x^2=100 q^2+20 q r+r^2$
$r^2=10$ (an odd number) $+$ a number less
than $10$
$r=16$ or $36$
$r^2=10+6$ or $3(10)+6$
Hence, the amount left for Madan at the end is $6$ rupees.
