Answer
$a_1=20
\\a_2=10
\\a_3=5
\\a_4=2.5
\\a_5=1.25$
Work Step by Step
Multiply the common ratio to get the next term to obtain:
$a_1=20
\\a_2=20(\frac{1}{2}) = 10
\\a_3=10(\frac{1}{2})=5
\\a_4=5(\frac{1}{2})=2.5
\\a_5=2.5(\frac{1}{2}) = 1.25$