A shopkeeper gains the cost of $8$ metres thread by selling $40$ metres thread. Find his gain percent.

A person earns $15 \%$ on investment but loses $10 \%$ on another investment. If the ratio of the two investments be $3: 5,$ what is the gain or loss on the two investments taken together in $\%$?

A man purchased $120$ reams of paper at $Rs.\, 80$ per He spend $RS.\, 280$ on transportation, paid octrol at the rate of $40$ palse per ream and paid $Rs.\, 72$ to a porter. In order to gain $8 \%$ he must sell each ream of paper for (in $Rs.$)

The percentage profit earned by selling an article for $Rs.\,1920$ is equal to the percentage loss incurred by selling the same article for $Rs.\,1280.$ At what price should the article be sold to make $25 \%$ profit? (in $Rs.$)

A shopkeeper gains $17 \%$ after allowing a discount of $10 \%$ on the marked price of an article. Find his profit percent if the article is sold at marked price allowing no discount.

By what fraction selling price $(S.P.)$ must be multiplied to get the cost price $(C.P.)$ if the loss is $20 \% ?$