Question

Logs are stacked up in apile as shown in the figure. The top row has 15 logs and the bottom row has 21 logs. How many logs are in the stack?

a1= 1+14=15
an= 7+14=21
s1=(n/2)(a1+an)

(n/2)(15+21) = 18

How do I figure out the n?
Would n be 14?
n/2(26) = 18
26n = 36
n = 36/26

something very wrong with your equation, since n has to a whole number

where does the 18 come from?
you seem to be using the formula
S(n) = n/2(first + last)
so 18 would be the sum of the logs ????
that makes no sense, since the bottom row alone contains 21 logs.
No

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