Answer
No. of otters born in 3rd year: $12$
Total No. of otters born in first 3 years : $17$
Work Step by Step
Given
$a_{n}= (n-1)(n+3)$
$a_{1}= (1-1)(1+3) =0 \times 4 = 0$
$a_{2}= (2-1)(2+3) =1 \times 5 = 5$
$a_{3}= (3-1)(3+3) =2 \times 6 = 12$
No. of otters born in 3rd year $(a_{3})=12$
Total No. of otters born in first 3 years :
$S_{3}=a_{1} + a_{2} + a_{3} $
$S_{3}= 0+5+12$
$S_{3}= 17$
