If Sarah wanted her total payments to be even lower, what could she do?

There are to types of sequence , an arithmetic sequence and geometric sequence. an arithmetic sequence has a common difference, while a geometric sequence has common ratio.
fist let us try if they have a common difference
D = a2 - a1 = ( 13 4/5) - ( 14 3/10 ) = - 1 / 2

D = a3 - a2 = ( 13 3/10) - ( 13 4/5) = - 1 / 2
 so they have a common difference then it is a arithmetic sequence