Please help its just 1 question

Cassie is traveling 675 miles today to visit her brother and his family. so far, she has stopped for gas three times. at the first stop, she put in 7.1 gallons of gas and paid $23.79. at her second stop, she put in 5.8 gallons of gas and paid $19.08. and, at the third stop, 6.6 gallons and paid $20.39. at which stop did casey pay the most for gas?

1/2+1/6-3/4 simplify the given expression leaving the answer in improper fraction form.

Find the amount of simple interest earned for depositing the given principle in an account if $2200 is invested at 5.5 % for 6 months