(a)Start with the students who scored from 0-50 marks, inclusive. "x students scored less than 51 marks". So x students represents all the marks from 0-50. Now go to the students who made from 50-100 marks, inclusive. "2x students scored more than 49 marks". So 2x students represents all the marks from 50-100. We've counted the number of students who have scored 50 marks twice though, so we need to make note of the info the problem gave and subtract out one of these times we double counted.
So total number of students:
And we also know that the total number of students is: