In a flower bed there are 23 rose plants in the first row, 21 in the second, 19 in the third and so on. There are 5 rose plants in the last row. Then the number of rows in the flower bed is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 10
Given:
- First row: a = 23
- Second row: 21
- Third row: 19
- Last row: l = 5
- Common difference: d = 21 - 23 = -2
We need number of rows n.
AP formula for nth term
$l = a + (n-1)d$
$5 = 23 + (n-1)(-2)$
$5 = 23 - 2(n-1)$
$5 = 23 - 2n + 2$
$5 = 25 - 2n$
$2n = 25 - 5 = 20$
$n = 10$