Use the rule for multiplication by power of 10 to caculate 20 × 100

To multiply by powers of 10, just add the zeros to the number you're multiplying it by. As a simpler example, 5 x 10 is 50; just add a zero to 5.  20 x 100 is 2,000 because you add the 2 zeros of 100 to the 20. I hope this helps!