20 multiplied by 50 is not too hard to compute in your mind. All you need to do is:
First, multiply the two first numbers, 2 and 5.
They make 10, so you have the first two numbers of your answer.
After that, because 20 has one zero and 50 also has one zero, you add the number of zeros at the end of each number to get two zeros in total. These make the last two numbers.
Once you have the first two (10) and the last two (00) numbers, you put them together to get:
20 * 50 = 1000
Hope this helps :)