Assume that the monthly food expenditures of families of a certain size in one economic group are normally distributed
with a mean of $130 and a standard deviation of $20. What
percentage of the expenditures are more than $90

The answer to that is 6

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.