Calculating the value of the missing item.

Sometimes you are given the average of a set of items and the value of all of the items except one. How do you find the value of the missing item?

All you need to do is add the value of the items you are given. Then subtract this from the total value of the items. Look at the examples below.

Example #1: The average weight of five boys is 80 pounds. The first four boys weigh 78 pounds, 87 pounds, 82 pounds and 79 pounds. How much does the fifth boy weigh?

Total weight of the five boys = 80 pounds x 5 = 400 pounds

Total weight of four boys = 78 + 87 + 82 +79 = 326 pounds

Weight of the fifth boy = 400 pounds – 326 pounds = 74 pounds

Therefore the weight of the fifth boy is 74 pounds

Example #2: The average amount of money earned by Zanika each day from Monday to Friday is $60. She earned $45 on Monday, $70 on Tuesday, $35 on Wednesday and $65 on Thursday. How much did she earn on Friday?

Total amount of money earned in five days= $60 x5 = $300

Money earned in four days = $45 + $70 + $35 + $65 = $215

Money earned on Friday = $300 – $215 = $85

Example #3: Ke’Andre did 3 sets of pushups. His average number of push-ups per set is 15. If he did 20 push-ups and then 17 push-ups for his first 2 sets, how many push-ups did he do for his third set?

Total number of pushups = 15 pushups x 3 = 45 pushups

Number of pushups for first 2 sets = 20 + 17 = 37 pushups

Number of pushups for third set = 45 – 37 = 8 push-ups