# Chapter 12 Sequences and Series - Chapter Review - Page 841: 23

$79.375$.

#### 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: $S_n=40(\frac{1-0.5^7}{1-0.5})=79.375$.

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