The alkene $\mathrm{CD}{}_3-\mathrm{C}(\mathrm{CH}{}_2\mathrm{CH}{}_3)=\mathrm{CH}-\mathrm{CH}{}_3$ is drawn with the $\mathrm{CD}{}_3$ group and the $\mathrm{CH}{}_3$ on the other carbon on the same side of the double bond. Its CIP descriptor is

Compare the atoms attached to the first carbon of each group: $\mathrm{CD}{}_3$ gives $(\mathrm{D},\mathrm{D},\mathrm{D})$ and ethyl gives $(\mathrm{C},\mathrm{H},\mathrm{H})$. The first point of difference is the highest member of each set: $\mathrm{C}$ (Z = 6) beats $\mathrm{D}$ (Z = 1). Isotopic mass is used only when everything else ties, so ethyl outranks $\mathrm{CD}{}_3$. The $\mathrm{CD}{}_3$ is cis to the $\mathrm{CH}{}_3$, so ethyl is trans to it: $E$.