If Jo saves $40 for n weeks and $50 for the next 8 weeks, the total savings = 40n+400 dollars.
An arithmetic sequence has only one common difference throughout the series, but this sequence has at least two: 40 for several weeks then 50 for 8 weeks, then presumably back to 40. This is a piecewise function. f(x)=
{ 40x | 0≤x≤n
{ 50x-10 | n<x≤n+8 because 40n+50(x-n)=50x-10n
{ 40x+80 | n+8<x because 80=8(50-40)
