Answer:
your answer is 12
Step-by-step explanation:
third term is 15, fourth term is unknown, call it "n", sixth term is n/2
_ _ 15 n _ n/2 is the sequence so far
d is the difference between terms
15+d=n
n+d=y where y is the fifth term
y+d=n/2
using the last two equations and substituting...
n+d+d=n/2
n+2d=n/2
2d=-n+n/2
2d=-n/2
d=-n/4
going back to the first equation (15+d=n) and substituting
15+(-n/4)=n
15=n+(n/4)
15=(5/4)n
multiply both sides by 4/5
(4/5)(15)=(4/5)(5/4)n
12=n
if the third term is 15 and the fourth term is 12, then d=-3 (or use the equation 15+d=n)
the sequence is:
21, 18, 15, 12, 9, 6
