Answer:
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.
_____
<em>Comment on the problem</em>
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 <em>the larger of the counts</em> is (70+20)/2, <em>half the sum of the given numbers</em>. This is generically true of all <em>sum and difference</em> problems.
Knowing this, you can work most of these problems in your head.
