Arnold started getting an allowance at his tenth birthday. Each month he got three times as many dollars as he was years old. If he saved 25% of his allowance, how much money would he
have saved by the day before his 18th birthday
eleventh year 12*10
twelfth year 12*11
...
seventeeth year 12*16
eighteenth year 12*17
12( 10+11+12 .... +17) (.25)
=12 (13.5*8)(.25)
= 324