Since you already know that about 3 out of 500 balls are defective and your trying to find out what amount of 100,000 balls are defective. Next you divide 100,000 by 500 to get 200. Now you know that 200 batches of 500 go into 100,000. Then you multiply 3 by 200 to find the estimate of the number of defective rubber balls this week. There are about 600 defective balls this week. then subtract 600 from 100000 to get 99400
B. Because the divisor, 4.2, has only value in the ones and tenths places, so we only have to multiply by 10 to get to 42, which is the whole number you need.