One hundred dollars is invested in 90 days. At the end of the term, the investment is worth $120. Find the growth rate per day.

I do NOT get this...

My equation was; 120=100 times b^90. Using the formula y= a times b^x Is that equation right, and how do I solve it? Even if it's not right, just PLEASE help me solve it. Thanks!

Oh, and if/when you do solve this, EXPLAIN? Thanks, cause, I've got 5 more just like this one to solve.