Answers

2016-07-13T19:43:50+08:00
Sum of arithmetic sequence =  \frac{a_1+a_n}{2} *n

We have a_1 = 30, d = 5, a_n = 30 + 5(n - 1).

We simplify a_n.
a_n = 30 + 5(n-1) = 30 + 5n - 5 = 5n + 25

We then substitute to formula,
500  =  \frac{30 + 5n + 25}{2} * 5 \\ 20 = \frac{n + 11}{2}  \\ 40 = n + 11 \\ 29 = n

Since you did not give the deadline, given the above problem it would take her 29 days to finish it.


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