Question 1202009
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If you just really, REALLY love magic formulas and don't care about actually understanding how to solve the problem, use the method shown by tutor @josgarithmetic.<br>
If you want a typical formal algebraic solution, use something like the method shown by tutor @ikleyn.<br>
But if a formal algebraic method is not required, and if the speed of obtaining the solution is important, then here is a quick and easy informal method for solving any problem like this mentally.<br>
(1) If all 80 cars had been small cars, the cost would have been 80($5) = $400; but the actual cost was $550, which is $150 more.
(2) The cost for each large car was $8-$5 = $3 more than for each small car.
(3) The number of large cars, to make the additional $150, was $150/$3 = 50.<br>
ANSWER: 50 large cars; 30 small cars<br>
CHECK: 50($8)+30($5) = $400+$150 = $550<br>