An aquarium shop had goldfish and guppies in the ratio 3 : 5 at first. There were 120 fish altogether at first. The shop sold 15 goldfish and bought some more guppies. After this, the ratio of the number of goldfish to the number of guppies became 1 : 3.
- How many goldfish were there at first?
- How many guppies did the shop buy?
Need a hint?
Find the number of goldfish at first first, then find the number of guppies needed to make the new ratio 1 : 3 with the goldfish left.
Worked answer
- Total 120 is shared into 3 + 5 = 8 parts, so 1 part = 120 ÷ 8 = 15.
- Goldfish at first = 3 × 15 = 45.
- Guppies at first = 5 × 15 = 75.
- After selling 15 goldfish, there are 45 − 15 = 30 goldfish left.
- The new ratio is 1 : 3, so guppies in the end = 3 × 30 = 90.
- Guppies bought = 90 − 75 = 15.
Answer: 15 fish
The mistake this question tests
Habit
Stopped at one part in ratio
Pupils often take the final number of guppies as 3 × 30 = 90 and stop there, forgetting to subtract the 75 guppies the shop already had.
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See the answer →Practise PSLE ratio until the mistakes stop.
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