Question

Mohan gets $3$ marks for each correct sum and loses $2$ marks for each wrong sum. He attempts $30$ sums and obtains $40$ marks. Find the number of sums solved correctly.

Answer 1

Let the sums attempted correctly be $x$ and the sums attempted wrongly be $y$.
$3x-2y=40$ ....(i)
$x+y=30$ .....(ii)
Multiplying (ii) by $3$
$3x+3y=90$ .....(iii)
Subtracting (i) from (ii), we get
$5y=50$

$\Rightarrow y=10$

$\therefore x=20$