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August 10th, 2017, 01:55 PM  #1 
Newbie Joined: Aug 2017 From: England Posts: 1 Thanks: 0  Help me please
I have a question: I have £1,000. I want to buy as many Magic stones as possible. Magic stones are £10. However each time you buy a Magic stone it's price increases by £4. £10,£14,£18... How many Magic stones could I buy? Please show how you work this out. 
August 10th, 2017, 02:26 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,399 Thanks: 546 
Cost of n stones = 10n+4(n(n1)/2). Set=1000 to find n, and take integer part of answer.

August 10th, 2017, 10:29 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 18,250 Thanks: 1439 
Type 10 + 14 + 18 + 22 + ... into Google until the running total would exceed 1000.

August 10th, 2017, 11:55 PM  #5 
Math Team Joined: Jul 2013 From: काठमाडौं, नेपाल Posts: 872 Thanks: 60 Math Focus: सामान्य गणित 
If you could buy 'n' magic stones $\displaystyle 10 + 14 + 18 + 22 .........n\hspace {2mm}times$ $\displaystyle (10+4×0) + (10+4×1)+ (10+4×2) + (10+4×3)... ....... n\hspace {2mm}times $ $\displaystyle [ 10+ 10+10+ 10+ .......... n\hspace{2mm} times] +[4×0+4×1+4×2+4×3 .......... n\hspace{2mm} times] $ $\displaystyle 10n +4 [0+1+2+3+........... n\hspace{2mm}times] $ $\displaystyle 10n +4 \frac {(n1)×n}{2}$ Equate this with 1000 and rounddown the answer 