Let the number of correct answers be x and the number of wrong answers be y.
Total questions Jayanthi answered = 120
So, x + y = 120 .....(i)
Now, marks obtained for answering correctly = 1 × x = x
Marks deducted for answering incorrectly = 1/2×y = y/2
Total marks obtained = x-y/2=90 .....(ii)
Subtracting (ii) from (i), we get
y+(y/2)=30⇒3y/2=30⇒y=20
Putting y = 20 in (i), we get
x + 20 = 120
⇒ x = 100
Thus, Jayanti answered 100 questions correctly.