Kaitlin worked on Transforming formulas. She practiced solving a given formula for a given variable. Then we learned to use formulas to solve problems. We learned to solve simple-interest problems. Then we learned that the rate for simple interest is the annual percentage rate (APR), or the percent used to calculate interest that is paid in one year. We learned that in simple interest calculations using the formula I = prt, t represents time as a fraction of a year.