First we add the numbers: 10,157 + 9987 = 20,144
To round, you take the hundreds number (in this case 1) and if it's 1,2,3,or 4, you round down, making the number just 20,000. If the hundreds number is 5,6,7,8, or 9, you round up, making the number 21,000. In this case, since the hundreds number is 1, we round down to 20,000.
Hope this helps!