about geometric progressions（G.P.）

Question

A gardener has a task to dig an area of 900 square meters first day, he digs an area of 10square meters For each subsequence day, he digs an area of 1.2times the area that he digs on the previous day until his task is completed. Find the number of days needed to complete the task. 

(THE ANSWER GIVEN IS 17 DAYS). 

Answer 1

for each SUBSEQUENT DAY...

 

10 + 1.2(10) + 1.2²(10) + ...

JUST WRITE OUT THE SERIES and as you go, develop the cumulative sum.

Observe when the total equals (or more likely, exceeds) 900.