SOLUTION: Kaye has an arrangement with her father where he pays her $5 for every maths test she passes and fines her $10 for every test she fails. Last year Kaye sat 9 tests and altogether h

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Question 810656: Kaye has an arrangement with her father where he pays her $5 for every maths test she passes and fines her $10 for every test she fails. Last year Kaye sat 9 tests and altogether her father paid her $15. How many tests did Kaye pass?
I must work this out with algebra but using a diagram I have worked out that she failed 2 and passed 7. Cna you help me with the algebra please. This is what I have tried so far:
Let p = pass and f = fail. f = -10 so p = f + 15
5p-10f=15
5(f+15) -10f=15
5f +75 -10f = 15
-5f + 75 = 15
(-75)
-5f = -60
(divided by -5)
f = 12
Please correct any of my working or feel free to start over :) Thanks so much in advance
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
p+f=9
p=9-f
5p-10f=15
5(9-f)-10f=15
45-5f-10f=15
-15f=-30
f=2
p=7
Check
7*5 -10*2=15