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

Solution 1.
Let the question solved correctly by Mohan = X ;
and number of question solved wrongly by Mohan = 30 - X ;
According to the question,
3X - (30 - X) x 2 = 40
⇒ 3x - 60 + 2x = 40
⇒ 5x - 60 = 40
⇒ 5x = 40 + 60 ⇒ x = 100/5 = 20
∴ Mohan attempted 20 questions correctly .
Solution 2.
Let the question solved correctly by Mohan = X ;
and number of question solved wrongly by Mohan = Y ;
According to given question;
X + Y = 30 ; -----------i
3X - 2Y = 40 ;-------ii
Now multiply the first equation by 2;
we will get
2X + 2Y = 60 ;--------iii
After Add the equation ii and iii , we will get
3X - 2Y + 2X + 2Y = 40 + 60 ;
⇒ 5X = 100 ;
⇒ X = 20;
∴ Mohan attempted 20 questions correctly .