SAT Right triangles Question

$$2\cos (90-a{)}^{\circ }\cdot \cos b^{\circ }+\sin (a+y{)}^{\circ }\cdot \sin (90-b{)}^{\circ }$$ In the given expression, $a$ and $b$ are constants such that $\sin a^{\circ }=0.50$ , $\cos b^{\circ }=0.99$ , and $0<b<90$ . What is the value of the given expression when $y=0$ ?