You math model is this system of two linear equations in two unknowns x + y = 80, (1) 90x + 80y = 6930. (2) Can you solve it ? Do you know how to solve it ? HINT: express x = 80 - y from (1) and then substitute it into (2) replacing x. You will obtain a single equation for y, which you can easily solve. This is "the Substitution method".