A boy buys 9 apples for Rs. 9.60 and sells them at 11 for Rs. 12. Find his gain or loss percent.

L.C.M of 9 and 11 = 99
Let 99 apples were purchased.
$\therefore$ C.P of 99 apples at the rate of 9 apples for Rs. 9.60
$=\frac{99}{9}\times 9.60=Rs.105.60$
S.P of 99 apples at the rate of 11 for Rs. 12
$=\frac{99}{11}\times 12=Rs.108$
$\because$ S.P > C.P
$\therefore$ Gain = S.P - C.P = Rs. 108.00 - Rs. 105.60
=Rs. 2.40
$Gain\%=\frac{Gain\times 100}{C.P.}=\frac{Rs.2.40\times 100}{105.60}=\frac{240\times 100\times 100}{10560\times 100}=\frac{100}{44}=\frac{25}{11}=2\frac{3}{11}\%$