posted by Beth .
Bill is saving money in an empty jar. The first day he puts in a certain amount of money. Every day after the first he puts in 2 more dollars than the amount he put in on the previous day. After 10 days he has put 200 dollars in the jar. How much money did he pit in the jar on the first day?
Is there a more concise way of doing this than writing x+x+2+x+4+x+6.... Etc until 10?
use what you know about arithmetic progressions.
10/2 (2a+9*2) = 200
2a+18 = 40
2a = 22
a = 11
So, the numbers are 11,13,...29
5(11+29) = 200