He invested two unknown amount that add to $1200.
Let the amount invested at 9% be x.
Let the amount invested at 3% be y.
The amounts add to $1200.
x + y = 1200 First Equation
The profit from the 9% part is 0.09x.
The profit from the 3% part is 0.03y.
The two profits add to $78.
0.09x + 0.03y = 78 Second Equation
We have two equations in two unknowns, so we can solve this problem as a system of equations.
x + y = 1200
0.09x + 0.03y = 78
Multiply the first equation by -0.09 and add to the second equation.
-0.09x - 0.09y = -108
0.09x + 0.03y = 78
----------------------------
-0.06y = -30
Divide both sides by -0.06
y = 500
Now we substitute 500 into y in the first equation.
x + y = 1200
x + 500 = 1200
x = 700
Answer: The amount invested at 9% is $700
Wait a password with 4 letters using only 5? And the letters can be used more than once. I’m confused
I can’t explain it perfectly because I don’t know
But x = 20
