I the average score is 750 and standard deviation ( sigma ) is 50 points, then score 700 - 800 is between (750 - 1 sigma) and (750 + 1 sigma).
That is 34.1 % + 34.1 % = 68.2 % of the whole population.
2,000 · 0.682 = 1.364
We expect 1,364 students to score between 700 and 800 points.