Hi! Refer to the model in the image. :)

2/5 of them are boys implies that the 5 rectangles must all be the same size, so all rectangles are the same size.

Without drawing, 2/3 of the boys are good at math = 2/5 of those good at math are boys So the "units" of the 2/3 are the same "units" of the 2/5, since the numerators are equal, and hence the whole story can be represented by the same "units".

Thanks Shaoyang Brandon and Teo Kai Meng... You make it looks so simple....

Alternatively, here's my not-so-visual solution ☺

Top row: Boys Bottom row girls: Girls.

Wow....so simple....awesome...

Izam Marwasi You're welcome! :D

Thanks Raymond.. Good alternative...fully understood...

Hi. (55-25)÷(7-5)=30÷2=15 workers

15×5+55=75+55=130 mooncakes bought

Thanks

Welcome!

Hi. Hint: what is not spent is hence saved.

Jan spent : saved = 840 : 100u Feb spent : saved = 1134 : 85u (increase by 35% is 1134, decrease by 15% is 85u) 15u = 1134 - 840 = 294 1u = 19.6 100u = 1960 Salary = 1960 + 840 = 2800

In Jan, 35%=7/20=21/60 → Spending = 60u = $840 15%=3/20=21/140 → Savings = 140u So salary = 200u = 200×($840/60) = $2800.

Hi! Refer to the image below for an explanation. :)

Thanks all for the explanation

👍

Elegant! How you make the 35% and 15% the same by transforming to percentage to fraction is quite clever! 😋

35%spending=15%savings → spending : saving = 15:35 = 3:7 Salary = 3u+7u = 10u = $840÷3×10 = $2800

Not really an answer to why, but the method works for bigger numbers too. I like this series of videos for math tips and tricks: https://youtu.be/SEupqNi2Rh8

Hi, does this help?

This method works best for digits close to 10, for digits 5 or below, this method will still work, but caution will have to be exercised as there will be "carry over" Eg for 8 × 4, a=2, b=6 So 8-6=2 (tens place) And 2×6=12 (ones place) 20 +12 _____ 32

In fact, this similar trick is a classic mental trick to multiply ninety something by ninety something. Hope this is useful.

Yes, it is indeed like this...looks like it works for all integers...thank you, Kai Meng!

Welcome!

Yes indeed! As Kai Meng has proven, the numbers chosen can be any integer, I have even tried it when the subtraction is negative and it still works, but of course I won't be sharing with the primary school pupils

The difference, ie A - B is really the digits in the tens place.. 😛. Without this understanding, some pupils will do this: 12 + 2 instead of 12 + 20