Meredith starts a saving fund for her son. She deposits $10 on his first birthday, $20 on his second, $30 on his third and so on....for 40 years (wow, is he still going to be living with her, too? Lol *ahem*....).

Use $\displaystyle t_{n}=a+(n+1)d$ to determine how much she will have deposited in total (ignore interest at this point).

I don't understand how to plug this into the formula, example:

$\displaystyle t_{3}=10+(30+1)(10)$, which equals $320, but logic tells me that $10 + $20 +30, only equals $60.

What am I missing?