20x1 and 20W7 how to read?

Hi, Don't worry too much about this as you will not see it in the exam and only ever see it in the study materials. The X and W relate to the decade effectively, so 20W0 to 20W9 is one decade and the next one would be 20X0 to 20X9, as X comes after W in the alphabet. In each decade the years then increase as normal so 20W1 is after 20W0, and then comes 20W2. Thanks