Need help in Probability question from Problem Solving part

GRE Quant Problem Solving
1

Sam completed a multiple-choice test with four options for each question. He was 90% sure of his answer on some of the questions, but on others in which he wasn’t 90% sure, he randomly guessed. If there were a total of 20 questions, and he got 11 right, what was the most likely number of questions that he was 90% sure on?

Round your answer to the nearest integer.

2

Let x be questions he was 90% sure on.
Thus he was unsure on (20-x) questions.
For unsure questions probability of being right is 25%. (Since it is an MCQ with 4 options)

Set up an equation as follows:
90x + (20-x)*25 = 11

On solving we get, x=9.23
x is integer, thus x=9

Cross check it,
9(0.9) + 11(0.25)
= 8.1 + 2.75
= 10.85 approx 11