A retailer marks his goods $25\%$ above $CP$ and also gives a discount of $10\%$. Find his gain $\%$.

Find Simple interest for the following:

$68000$ at $16\frac{2}{3}\%$ per annum for $3$ years.

Find Simple interest for the following:

$3000$ at $18\%$ per annum for $\frac{3}{2}$ years.

Find Simple interest for the following:

$155000$ at $6\%$ per annum for $5$ years.

Find Simple interest for the following:

$6250$ at $4\%$ per annum for $36$ months.