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 August 10th, 2017, 12:55 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, 01:26 PM #2 Global Moderator   Joined: May 2007 Posts: 6,806 Thanks: 716 Cost of n stones = 10n+4(n(n-1)/2). Set=1000 to find n, and take integer part of answer.
 August 10th, 2017, 03:47 PM #3 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,958 Thanks: 1146 Math Focus: Elementary mathematics and beyond See here.
 August 10th, 2017, 09:29 PM #4 Global Moderator   Joined: Dec 2006 Posts: 20,926 Thanks: 2205 Type 10 + 14 + 18 + 22 + ... into Google until the running total would exceed 1000.
 August 10th, 2017, 10:55 PM #5 Math Team     Joined: Jul 2013 From: काठमाडौं, नेपाल Posts: 901 Thanks: 61 Math Focus: सामान्य गणित If you could buy 'n' magic stones $\displaystyle 10 + 14 + 18 + 22 .........n\hspace {2mm}times$ $\displaystyle (10+4×0) + (10+4×1)+ (10+4×2) + (10+4×3)... ....... n\hspace {2mm}times$ $\displaystyle [ 10+ 10+10+ 10+ .......... n\hspace{2mm} times] +[4×0+4×1+4×2+4×3 .......... n\hspace{2mm} times]$ $\displaystyle 10n +4 [0+1+2+3+........... n\hspace{2mm}times]$ $\displaystyle 10n +4 \frac {(n-1)×n}{2}$ Equate this with 1000 and round-down the answer

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