Hi all i have these two problems that have been bothering me , any 1 know how to solve it please

1. A string vibrates 30mm in the first oscillation and subsequently 90 % of its previous value in each succeding ocsillation. How many oscillation will it take before the vibration falls below 10mm and how far will it have vibrated in this time?

2. The sum of the first n terms of a geometric series is

1/3 - 2^n/3^n+1

Find

ii) The first term

ii) the second term

iii) the common ratio