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on monday, revlien paid 66 pesos for 3 doughnuts and 2 cups of coffee.
on tuesday, she paid 69 pesos for 2 doughnuts and 3 cups of coffee.
how much did he pay for 1 doughnut and 1 cup of coffee on a wednesday?
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This problem is intended for mental solution.
Add her purchases on Monday and Tuesday:
she bought 3+2 = 5 doughnuts and 2+3 = 5 cups of coffee, in total,
spending 66 + 69 = 135 pesos.
HENCE, one doughnut and one cup of coffee cost 1/5 of it, i.e. 135/5 = 27 pesos.
ANSWER. On Wednesday, she must pay 27 pesos.
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Solved.
Notice that I did not solve any systems of equations.
I even did not write them.
It is a classic type of problems, that seem to be solved
using equations, but actually MUST be solved mentally.
Solving mentally is a GOOD STYLE solving such problems.
It is what the teachers expect to get from their students,
when they give them such problems.
It is what the interviewers expect to get from the candidates
when they interview them somewhere in Microsoft or Google.
But the final goal of teachers, when they give such problems, is to develop
the student's mind FLEXIBLE and CREATIVE, instead of making it dogmatic.