Bob is a lovely schoolboy. He likes playing football very much. He often plays football with his classmates after school, but he never goes home too late. He usually gets home between 4:30 and 5:30 in the afternoon. One day he comes back home late, so his mother says, ”You come back home late today, Bob!””Yes, we have got a new teacher. He teaches us Maths,” Bob answers. “How is he?”his mother asks. “I don't know. I think I can't believe him.””Why?”his mother says. “In today's Maths class, one moment he says two and three is five. But the next moment he says one and four is five.” Bob answers.

如图，三角尺在灯泡O的照射下在墙上形成影子，现测得OA=20cm， =50cm，则这个三角尺的面积与它在墙上所形成影子图形的面积之比是____。