Please show all work and don't use excel functions to solve.

1. The IRS reports that the mean refund for a particular group of taxpayers was $1,600. The distribution of tax refunds follows a normal distribution with a standard deviation of $850.

a. What percentage of the refunds are between $1,600 and $2,000?

b. What percentage of the refunds are between $900 and $2,000?

c. What percentage of the refund are between $1,800 and $2,000?

d. Ninety-five percent of the refunds are for less than what amount?