Answer:
S=252logs
Step-by-step explanation:
bottom layer has a1 = 25
second layer has a2 = a1 - 1 = 24
third layer has a3 = a2 - 1 = a1 - 2 = 23
.
.
n-th layer has an an = a1 - n = 25 - n
the top layer contains an = 25 - n and we know that the top layer has 11 logs thus
25 - n = 11
n = 25 - 11
n = 14
there are 14 layers
the total number of logs is the sum of the first 14 members of the Arithmetic Progression 25, 24, 23, 23,.....11 where
an = 25 - n
Sn = (n/2)(a1 + an) and since n = 14
S14 = (14/2)(25 + 11) = 7*36 = 252
S14 = 252 logs
