Answer
$75$
Work Step by Step
From part (a), we know that $a_n= a_1 r^{n-1}$ ...(1)
Here, we have Common Ratio $r=2$ and first term $a_1=5$
Equation (1) gives: $a_n=5 \times (2)^{n-1}$
We know that $S_{n}=a_1(\dfrac{r^{n}-1}{r-1})$
Now, $S_{4}=5[\dfrac{1-(2)^{4}}{1-2}]$
$S_{4}=5[\dfrac{1-16}{-1}]$
This gives: $S_{4}=5 \times \dfrac{-15}{-1}$
Hence, $S_{4}=75$
