Given that

, we have for

the Taylor series expansion about 0 as

Replace

with

, so that the series is equivalent to

and notice that

Recall that for

, we have

which means

Based on the numbers we have we can assume that she saves 3 times more each week than the last (1*3=3, 3*3=9).
Following this trend we would multiply the amount she saved the third week ($9) by 3 to get $27 for the fourth week.
Then, we would multiply the amount she saved the fourth week ($27) by 3 to get $81 for the fifth week.
Finally, to figure out how much she saved in the 5 weeks, we need to add each value up to get 1+3+9+27+81= $121 saved in 5 weeks
Since there are 5 possible outcomes and 3 is only one of the outcomes, the probability would be 1/5 or 20%