Find the 21st term of the AP: 3.4, 3.1, 2.8, 2.5, ...

$a=3.4,d=-\mathrm{0.3.}{a}_{21}=3.4+20\ast (-0.3)=3.4-6=-2.6$. Using 21 steps gives -2.9 and 19 steps gives -2.3; 9.4 comes from adding 6 instead of subtracting it, but this AP is decreasing.