Description: Draw a normal distribution curve first.

Since the values of 650 and 850 are at 60% and 90%, they must be on the right half curve. First of all, you have to figure out 60th? Percentile refers to the concept that the total probability of taking any value on the left of a point is 60%, and the ordinate represents the probability and the abscissa represents the value, that is, the integral of the ordinate values of all micro-divisions on the left of 650 points is its percentile percentage.

Ok, now you mark the abscissa position of 650 and 850 on the X axis, and then find the midpoint of its abscissa, which is 750. Then you connect the points corresponding to 650 and 850 on the curve with line segments and find the middle point. Obviously, the abscissa of this point is also 750, but its ordinate is larger than the point corresponding to 750 on the curve. You know everything here.

We can obviously find that the vertical integral of the curve between 650 and 750 is greater than that of the curve between 750 and 850. It shows that when moving from 650 to 750, its percentile value not only moves from 60% to 75% (the midpoint between 60% and 90%), but also exceeds 75%, so the percentile value corresponding to 750 exceeds 75%. Then obviously, the corresponding value of 75% must be on the left of 750 (because the percentile value must increase to the right), so it is less than 750.