Question

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Answer 1

Answer:

• Todd: 25
• Andrew: 45

Step-by-step explanation:

We can let "a" stand for the number of goldfish Andrew has. Then the number Todd has is (70-a). We are told the difference is ...

  a - (70-a) = 20

  2a = 90 . . . . . . .  .collect terms, add 70

  a = 45

  70-a = 25

Andrew has 45 goldfish; Todd has 25.

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Comment on the problem

I call this a "sum and difference" problem. You are given the total of two counts, and also their difference. If you were paying attention to the above solution, you see that the larger of the counts is (70+20)/2, half the sum of the given numbers. This is generically true of all sum and difference problems.

Knowing this, you can work most of these problems in your head.