Answer
$79.375$.
Work Step by Step
The sum of a geometric sequence until $n$ can be obtained by the formula $S_n=a_1(\frac{1-r^n}{1-r})$ where $a_1$ is the first term and $r$ is the common ratio.
Here $a_1=40,r=0.5,n=7$
Hence here the sum is: $S_n=40(\frac{1-0.5^7}{1-0.5})=79.375$.
