Answer: He used 200 Blocks.
Step-by-step explanation: This question is a classic volume problem. Remember that the formula for Volume is B(Area of Base) x h(height). We can find the area of the base by multiplying the length(20) by the width(10) giving us a total area of base as 200. Now all that is left to do is find the height and multiply it with the area of base. We can see this by counting how many layers there are, in this case 1. So, plugging this into the formula for volume we get 200 x 1 which gives us a total of 200 blocks used.