The ratio of the number of goldfish to guppy in a fish tank is 5:4. After 3 guppies died, the ratio of the number of goldfish to guppy is 4:3. What was the total number of angelfish and clownish in the aquarium at first? [2]

How to solve this question?

First of all, the student has to identify the topic. In this question it is clear cut, and the topic in question is obviously ratio.

Next, what technique to use? Ratio is a concept introduced in P5, and as such, students are typically motivated to use models. However, the ratios given will make modeling difficult, as the numbers are very close to one another, and there has been a subtraction somewhere down the road.

So, how do we address this issue? For most questions on ratio, I would use my ratio method! The method is simplistic and easy to visualize.

Before:                 After:

GF:GP                   GF:GP

5:4                          4:3

Note that there is one variable that remains constant! That is the value of the goldfish. Since the total goldfish does not change, we can hold that value constant. This means, that the value of the goldfish before and after will remain the same. Based on what the student knows about ratio, the student must recall that you can expand a ratio beyond its simplest form. Since we want the ratio or the units of goldfish to be uniform for before and after, we shall expand by multiplying the ratio in before by x4, and multiplying the ratio in after by x5.

Before:                 After:

GF:GP                   GF:GP

20:16                     20:15

As you can see, now the ratio of goldfish is uniform, and it makes it so much easier to compare! The guppies have decreased from 16 units to 15 units, and goldfish remains the same, this checks out with regard to the question. Next, the question says that 3 guppies died, so the decrease of 1unit has to be representing the death of the 3 guppies!

Therefore, 1 unit for both ratio represents 3, and since there are 20+16=36 units at first, the answer is simply 36×3=108!

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DETAILED SOLUTION TO PSLE SCIENCE TYS QUESTION(question adapted due to copyright issue)

Question:

For some buildings, water mist systems are used to cool the surrounding air, and tiny water droplets are produced in the form of mist, as shown in the figure

1. Explain how such a system is able to lower the temperature of the surrounding air and give a reason for your answer. [2]

2. If you were to add fans to the water mist system, how would it help to cool the surrounding air more effectively? [1]

Answer:

How to tackle PART 1?

First off, we identify the topic in question. Based on the keywords used which is mist, and temperature, the student has to identify that this is a multi-topic question, and topics involved are water cycle and heat, specifically, on the concepts of the 3 states of water and temperature.

Next, recall the information learnt from these topics. Water has three states, solid, liquid, gas, and different processes convert water into its individual forms.

So, the mist is used to lower the temperature of the surrounding air. The student has to be able to recall that water is a cooling substance, and its different forms can act to cool environments. Based on the question, the mist form seems gaseous at some point, so the student must recall that evaporation results in gas, and during evaporation, water absorbs heat and changes into water vapor.

Now, we piece the clues together and form our solution.

The mist system will release mist, which are tiny water droplets, into the surroundings. The water droplets absorb heat from the surroundings and they evaporate. This cools the surrounding air, and the temperature of the surrounding air is lowered.

How to tackle PART 2?

Part 2 is what we like to unofficially call a creative question. These sort of open ended questions are follow-up questions to part 1, meaning, if you have failed to understand part 1, you will not be able to do part 2, and lose 3 whole marks. These questions are typically designed such that the answers cannot be found in the textbook, and to make the student think deeply about the subject matter. So how do we go about doing this?

By looking at the figure, we can see that there is now a fan behind the mist system. So what is the fan doing? The student must be able to identify that fans generate wind, and wind typically blows things around. But this is a layman term, we have to explain it in scientific terms.

So, what does the act of wind blowing things around cause? It causes things to move further away. Based on part A, the student has to realize that the purpose of the mist system is to cool the surroundings. So what does blowing the mist further away achieve? At this point, the student has to come to the conclusion, that blowing it further away covers more ground, and cools more people! Now we piece the clues together and form our solution.

The fans blow the mist further away to other nearby places, and this cools the other places as well, because now the mist covers a larger surface area.

Stay tuned for more solutions to popular promo and TYS questions!

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Mr Y.S Pang, B.Eng(hons) NUS, Science tutor, Singapore Learner and Concept Learning

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