$\begin{aligned}
& \mathrm{S}3=3 \mathrm{a}+3 \mathrm{d}=54 \
& \Rightarrow \mathrm{a}+\mathrm{d}=18 \
& \mathrm{S}{20}=10(2 \mathrm{a}+19 \mathrm{d}) \
& \Rightarrow 10(36+17 \mathrm{d}) \
& \Rightarrow 1600 < 10(36+17 \mathrm{d}) < 1800 \
& \Rightarrow 160 < 36+17 \mathrm{d} < 180 \
& \Rightarrow 124 < 17 \mathrm{~d} < 144 \
& \Rightarrow 7 \frac{5}{17} < \mathrm{d} < 8 \frac{8}{17}
\end{aligned}Commondifferencewillbenaturalnumber\begin{aligned}
& \Rightarrow d=8 \Rightarrow a=10 \
& \Rightarrow a_{11}=10+10 \times 8=90
\end{aligned}$