If your school have 255 students, about how many would you expect to have an iq less than 70?

6

Step-by-step explanation:

Assuming a mean of 100 and standard deviation of 15, 70 is 2 standard deviations below the mean.

According to the empirical rule, 95% of the population is within ±2 standard deviations.  Which means half of that, or 47.5%, is between 0 and -2 standard deviations.  Since 50% of the population is below 0, then 2.5% is below -2 standard deviations.

2.5% of 255 is approximately 6.