Assumption method :


Assume all 80 are adults first.

Number of plates of noodles eaten
= 80 x 2
= 160

Now that is too many.
160 - 85 = 75

We need to reduce the number of plates eaten by 75. We do this replacing some adults with same number of children.
This keeps the total number of people the same but reduces the plates eaten.

Now we know every 3 children share 1 plate.

So if we replace 3 adults with 3 children,
3 adults means 3 x 2 = 6 plates
6 - 1 = 5

So there is a decrease in 6 plates as 3 adults are removed, but then there's a 1 plate increase as 3 children are added.
So there's an overall decrease of 5 plates for every 3 adults replaced.


For 75 plates,

75 ÷ 5 = 15

We need to remove 15 sets of 3 adults.
15 x 3 = 45

45 adults are removed and this also means 45 children replaced them.

So total adults = 80 - 45 = 35
Children = 45

Guess and check is also ok, but can be tedious at times and takes up much time in exams.

I will recommend assumption method since it's more logic based and is usually much quicker.

Ok,thank you!

I didn’t know that “every adult” can eat an amount of bee hoon equal six times the amount “every child” can eat. The discrepancy is quite large in a real situation context.

Well it depends on how young the children are... if you have 3 toddlers aged 5 then ⅓ of a plate is just nice. As for adults, 2 plates is typically too much unless it's Caucasian/westerner appetites


And I think the 3 is introduced to make it harder for students, i.e can't assume with children first as 80 ÷ 3 would not be give whole number for plates of beehoon