Matthew Fan's answer to iliketostayathome's Primary 6 Maths Ratio Singapore question.
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Besides using the guess and check method, here is one way to get values that work. I want 91 to be expressed as the sum of some multiples of 2, 5 and 10.
How many 2s must I have? I need to have some such that after i subtract that many 2s from 91, im left with a multiple of 5(because im only left with multiples of 5 and 10, which are all multiples of 5)
How many 2s must I have? I need to have some such that after i subtract that many 2s from 91, im left with a multiple of 5(because im only left with multiples of 5 and 10, which are all multiples of 5)
Date Posted:
4 years ago
91 89 87 85 aha 85 is a multiple of 5. Now we can write 91=85+2*3. After that i have 12 notes of 5 and 10 left over and they add to 85.
If we assume all of them are 5 then it would be 12*5=60. So 85-60=25 tells me there is an excess of 25 which must come from the 10s. How many of them? 10-5=5 is the excess of each $10 note and excess of 25 divide by excess of 5 each gives 5.
So 5 notes of $10, and 12-5=7 notes of $5
If we assume all of them are 5 then it would be 12*5=60. So 85-60=25 tells me there is an excess of 25 which must come from the 10s. How many of them? 10-5=5 is the excess of each $10 note and excess of 25 divide by excess of 5 each gives 5.
So 5 notes of $10, and 12-5=7 notes of $5
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