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How many cards would be needed for a 65 storey house of cards if there was 3 on the top row with the number of cards on each storey increasing by 3 each time you go down one storey. Example: One storey of cards has 3 cards, 2 storeys have 9 cards and so on so forth

 May 1, 2014

Best Answer 

 #2
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sum_(n=1)^65 3 n = 6435

 

Note that we are summing 3(1) + 3(2) + 3(3) + ....+ 3(64) + 3(65)

 

Another way to see this is that the sum of the 1 st 65 positive integers = [(65)(66) / 2] = 2145

 

So we have    3 * (1 + 2 + 3 + .... + 64 + 65) = 3 * [(65)(66) / 2] = 6435

 


 

 

 
 May 1, 2014
 #1
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well by the formula you provided it would be 3 raised to the 65th power which is $${{\mathtt{3}}}^{{\mathtt{65}}} = {\mathtt{10\,301\,051\,460\,877\,537\,453\,973\,547\,267\,843}}$$ cards.

 May 1, 2014
 #2
avatar+130511 
+5
Best Answer
sum_(n=1)^65 3 n = 6435

 

Note that we are summing 3(1) + 3(2) + 3(3) + ....+ 3(64) + 3(65)

 

Another way to see this is that the sum of the 1 st 65 positive integers = [(65)(66) / 2] = 2145

 

So we have    3 * (1 + 2 + 3 + .... + 64 + 65) = 3 * [(65)(66) / 2] = 6435

 


 

 

 
CPhill May 1, 2014

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