The number of short students in a class is 2/5 of the total number of students. If there are 90 tall students, and the class has 400 students, calculate the total number of students with average height. Think carefully first, then make a decision:
The number of short students in the class is 2 / 5 * 400 = 160. The total number of short and tall students is 160 + 90 = 250. Since there are 400 students in the class, the number of students with average height is 400 - 250 = 150. So, the answer is 150.