If the ratio of the cost price and selling price of an article is $20: 21$,then what will be the profit percent (in $\%$)?

$A$ 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 $\%$)

$A$ cab driver makes a profit of $20 \%$ on every trip when he carries $3$ passengers and the price of petrol is $Rs. 30$ a litre. Find the percentage profit for the same journey if he goes with four passengers per trip and the price of petrol reduces to $Rs. 24$ a litre? (in $\%$)
(Assume that revenue per passenger is the same in both the cases.)

$A$ person bought a horse and a carriage for $₹ 20000$. Later,he sold the horse at $20 \%$ profit and the carriage at $10 \%$ loss. Thus,he gained $2 \%$ in the whole transaction. The cost price of the horse was (in $₹$)

$A$ dealer sold a bicycle at a profit of $10 \%$. Had he bought the bicycle at $10 \%$ less price and sold it at a price $Rs. 60$ more,he would have gained $25 \%$. The cost price of the bicycle was:

$A$ discount series of $10 \%$,$20 \%$,and $40 \%$ is equal to a single discount of.......$\%$