Answer
No. of species born in 4th year: $35$
Total No. of species born in first 4 years : $82$
Work Step by Step
Given
$a_{n}= (n+1)(n+3)$
$a_{1}= (1+1)(1+3) =2 \times 4 = 8$
$a_{2}= (2+1)(2+3) =3 \times 5 = 15$
$a_{3}= (3+1)(3+3) =4 \times 6 = 24$
$a_{4}= (4+1)(4+3) =5 \times 7 = 35$
No. of species born in 4th year $(a_{4})=35$
Total No. of species born in first 4 years :
$S_{4}=a_{1} + a_{2} + a_{3} +a_{4} $
$S_{4}= 8+15+24+35 $
$S_{4}= 82$
