Question 18
In the following diagram, $\triangle AOB$ is an isosceles right-angled triangle with $OA=OB=40$. Point D lies on the side OB such that $OD:DB=3:1$. Point E is a point on AD such that OE is perpendicular to AD and OE extended meets AB at C. If $[OAC]$ denotes the area of $\triangle OAC$, find the value of $7[OAC]$.
