Answer:
first term(a)= 80
common ratio(r)= t2/t1 =40/80 =1/2
T4= ar^n-1
= 80*1/2^4-1
=80*1/2^3
=80*1/8
=10
then
t5=ar^n-1
= 80*1/2^5-1
=80*1/16
=5
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