A grocer buys eggs at 10 for RS 8 and sells at 8 for Rs. 10. Find his gain or loss percent.

Cost price per egg $=\frac{8}{10}=0.8$
Selling price per egg $=\frac{10}{8}=1.25$
Gain$=1.25-0.8=Rs.0.45$
gain %$=\frac{0.45}{.8}\times 100=56.25$%.