The total amount of CD's are 100% so 100 - (20+60) = 20% -> the percentage of the other types of music. Then multiply 20% by 20 to get 4. So there are 4 CD's that are other types of music
its because the value of 1 in the ten place and multiply by 10 to get the 1 in the hundred place or the 1 in the hundred place divide by 10 to get 1 in the tens place