Ying lives at location A and works at location B as indicated below. Note that Ying needs to travel at least 20 blocks to go to work, namely 10 block south and 10 block east. Find a formula for the number of ways Ying can get from home to work (under the condition that she travels only 20 blocks). This is the diagram:
I know it isn't that hard but something about this just makes me stuck. I am thinking that the answer is 20C10 but I don't think it is correct, and how I would arrive to that.