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Algebra/Algebraic Equation


John completes a job in x hours, while Rachel completes the same job in (x+4) hours. If they work together, they can complete the job in 3 hours and 45 minutes.
By forming an equation in x, show that 2x^2-7x-30=0.

John: x hours to complete job means he works at a rate of 1/x.
Rachel: x+4 hours to complete the job means he works at a rate of 1/(x+4).

For both working together, they work at a rate of 1/x + 1/(x+4), and that take 3.75 hours.

This says we have 1/[1/x + 1/(x+4)] = 3.75, and 3.75 = 15/4, so we have.
1/[1/x + 1/(x+4)] = 15/4.

Inverting both side gives 1/x + 1/(x+4) = 4/15.

Multiplying both sides by 15x(x+4) gives 15(x+4) + 15x = 4x(x+4).

Multiplying out each side of the equation gives 15x + 60 + 15x = 4x + 16x.

Adding 15x + 15x on the left side gives 30x + 60 = 4x + 16x.

Adding -30x - 60 to both sides gives 0 = 4x - 14x - 60.

Dividing each term  by 2 gives 0 = 2x - 7x - 30.

That is what we are given to show.


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Scott A Wilson


Any algebraic question you've got. That includes question that are linear, quadratic, exponential, etc.


I have solved story problems, linear equations, parabolic equations. I have also solved some 3rd order equations and equations with multiple variables.

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