you should start with an equation.
[tex]245 = 125 + 15x[/tex]
The 'x' represents the week.
so now, we have to subtract 125 from the total amount which is 245.
[tex]245 - 125 = 120[/tex]
now we have
[tex]120 = 15x[/tex]
to find 'x', we must divide 15 on each side
[tex] \frac{120}{15} = \frac{15x}{15} [/tex]
we are now left with
[tex]8 = x[/tex]
that means Sierra will have 8 weeks until she has enough money to buy the bike
