Find a simple discount equivalent to a discount series of $10 \%, 20 \%$ and $25 \%$ (in $\%$).

$A$ trader buys two articles for $Rs. 4000$ each. While selling,if he gains $12.5\%$ on one and loses $20\%$ on the other,what will be the overall loss percent (in $\%$)?

$A$ vendor bought toffees at $6$ for a rupee. How many for a rupee must he sell to gain $20 \%$?

$A$ wholesaler had $200$ dozens of mangoes. He sold some of these mangoes at $20 \%$ profit and the rest at $10 \%$ profit,so that he made $13 \%$ profit on selling all the mangoes. How many mangoes (in dozens) did he sell at $20 \%$ profit?

In these questions,two equations numbered $I$ and $II$ are given. You have to solve both the equations and mark the appropriate option.
$I$. $6x^2 + 19x + 15 = 0$
$II$. $3y^2 + 11y + 10 = 0$

$A$ shopkeeper buys articles at $8$ for $Rs\,16$ and sells them at $9$ for $Rs\,20$. What is his gain or loss percent?