There are 100 questions in a question paper, each carrying 2 marks. But there is a negative marking. 1 mark is deducted for each question answered incorrectly. A student answering all the questions got 80 marks. The number of questions correctly answered by him is

Let the number of correctly answered questions be $x$.
Since the student answered all questions, the number of incorrectly answered questions is $100-x$.

Each correct answer gives $2$ marks and each incorrect answer deducts $1$ mark.
The total marks obtained is $80$. We can set up the equation:

$$2x-1(100-x)=80$$

Solving for $x$:

$$2x-100+x=80$$

$$3x=180$$

$$x=\frac{180}{3}=60$$

Thus, the number of questions correctly answered is $60$.