Hi
2 question i want check the answer .
thanks
You are correct that these are both questions about arithmetic series. But your formulas are incorrect so your answers are wrong. You need a better undersanding of what each term of the formulas actually do. Anyway, here are the correct formulas:
$\displaystyle a_n = a_1 + (n-1)d$
$\displaystyle S_n = \frac{a_1 + a_n}{2}(n)$
In both problems, you know the sum $\displaystyle S_n$ as well as $\displaystyle u_1$ and $\displaystyle d$. You can plug the first formula into the second to get rid of the $\displaystyle a_n$, which would make $\displaystyle n$ the only variable. In solving the equation for $\displaystyle n$, you should see that it is a quadratic equation: $\displaystyle 0 = \frac{d}{2}n^2 + (u_1 - \frac{d}{2})n - S_n$. The simpliest way to solve these will be by using the quadratic formula.
The answers I get are 20 months and 5 years.