Q78. Let $\overrightarrow { O A } = 2 \vec { a } , \overrightarrow { O B } = 6 \vec { a } + 5 \vec { b }$ and $\overrightarrow { O C } = 3 \vec { b }$, where $O$ is the origin. If the area of the parallelogram with adjacent sides $\overrightarrow { \mathrm { OA } }$ and $\overrightarrow { \mathrm { OC } }$ is 15 sq. units, then the area (in sq. units) of the quadrilateral OABC is equal to :\\
(1) 32\\
(2) 40\\
(3) 38\\
(4) 35