$A$ dishonest dealer uses a scale of $90 \, cm$ instead of a metre scale and claims to sell at cost price. His profit percentage is:

$A$ vendor buys bananas at $9$ for $Rs. 8$ and sells at $8$ for $Rs. 9$. What will be the profit or loss (in $\%$)?

An electric pump was sold at a profit of $15 \%$. Had it been sold for $Rs. 600$, the profit would have been $20 \%$. What was the former selling price in $Rs.$?

Mohit purchased $40 \, kg$ of wheat at $Rs. 12.50$ per $kg$ and $25 \, kg$ of wheat at $Rs. 15.10$ per $kg$. He mixed the two qualities of wheat for selling. At what rate should it be sold to gain $10 \%$? (in $Rs.$)

The ratio between the selling price and the cost price of an article is $7:5$. What is the ratio between the profit and the cost price of that article?

$A$ shopkeeper mixes $26 \, kg$ of wheat which costs $Rs. 20$ per $kg$ with $30 \, kg$ of wheat which costs $Rs. 36$ per $kg$ and sells the mixture at $Rs. 30$ per $kg$. Find the profit percent. (in $\%$)