To find out how much Sierra will have saved after 4 weeks, we can use the expression provided:
\[ \text{Amount saved} = 45 + 3w \]
where \( w \) represents the number of weeks.
Substituting \( w = 4 \):
\[ \text{Amount saved} = 45 + 3(4) \]
Calculating \( 3(4) \):
\[ 3(4) = 12 \]
Now, adding that to 45:
\[ 45 + 12 = 57 \]
So, Sierra will save $57.00 after 4 weeks.
The final answer is: Sierra will save $57.00 after 4 weeks.