Elena has bought 320 tropical fish for a museum display. She bought 7 times as many triggerfish as parrotfish. How many of each type of fish did she buy?

The total number of fish bought was 320. And you know she bought two types of fish. The best way to solve this is guess and check. It would be the fastest. So she bought 7 times as many trigger-fish as parrot fish that means the number of trigger-fish bought was 7p. This means 320 = 7p + p. So all you do is take educated guesses for the number of parrot fish and check if it is right. So if she bought 50 parrot fish. 7 (50) + 50 = 400. Close but a bit high. Lets keep guessing. 7 (35) + 35 = 245. So now we know the answer is between 50 and 35. So lets try 40. 7 (40) + 40 = 320. That works so we know she bought 40 parrot fish and 280 trigger-fish.