Logs are stacked so that there are 21 logs in the bottom row, 20 logs in the second row from the bottom, and so on, decreasing by 1 log each row.How many logs are there in the first four rows from the bottom?
A) 140 logs
B) 60 logs
C) 78 logs
D) 153 logs
E) 165 logs

Question

Quizplus · Accepted Answer

We can use a simple arithmetic series formula to find the total number of logs in the first four rows:
Total = (n/2) x (a + l)
where n is the number of terms, a is the first term, l is the last term, and all terms are added together.
In this case, n = 4 (since we want the first four rows), a = 21 (the first row has 21 logs), and l = 18 (the fourth row has 18 logs). 
Plugging these values into the formula, we get:
Total = (4/2) x (21 + 18) = 2 x 39 = 78
Therefore, there are 78 logs in the first four rows from the bottom. So, the best choice is (C).

Logs Are Stacked So That There Are 21 Logs in the Bottom