Question

David keeps records of his monthly earnings. Last month he worked 120 hours and earned $2700. This month he worked 100 hours and earned $2300. Which equation could you use to find f(x), the amount he would earn at this rate if he worked x hours?
f(x)=120x+400

f(x)=400x

f(x)=100x+300

f(x)=20x+300

and tell me why it's the answer.... please. ^-^"

Answer 1

F(x) = 20x+300
you sub in the x values (120 hours and 100 hours) for each equation
the only one they both work for is the last one
f(x) = 20x + 300
f(100) = (20 * 100) + 300 = 2000 + 300 = 2300

f(120) = (20 * 120) + 300 = 2400 + 300 = 2700

Answer 2

Substitute x in the equations for numbers of hours worked, and you should look for the one that gets the right amount of money.

If you use 120 hours, you should look for the equation that gives you the answer of 2700:

f(x)=120x+400
120(120) + 400 =14800

f(x)=400x
400(120) = 48000

f(x)=100x+300
100(120) + 300 =12300

f(x)=20x+300
20(120) + 300 = 2700

Just to check, use the other number of hours: 100 hours.. You should expect to see an answer of 2300:
f(x)=20x+300
20(100) + 300 = 2300

the equation you want is f(x)=20x+300