Problem: find the area of trapezium whose parallel sides are 30 cm and 18 cm long , and the distance between them is 15 cm .
Rationale: "area of a trapezium = 1 / 2 ( sum of parallel sides ) * ( perpendicular distance between them ) = 1 / 2 ( 30 + 18 ) * ( 15 ) = 360 cm 2 answer : c"
annotated_formula: quadrilateral_area(15, 18, 30)