Question

(5 pts) It takes Jack 3 hours longer to write holiday cards than it takes Jill. Together, they can write them
in 2 hours. How long does it take each person to write the cards working alone?​

Answer 1

Answer:

Jack needs 6 hours and Jill needs 3 hours.

Step-by-step explanation:

Let's define the variables:

X  = rate at which Jack can write a holiday card.

Y = rate at which Jill can write a holiday card.

Each of them will need a given time to write a card, such that:

X*T = 1 card

We know that Jack needs 3 more hours to write a card than Jill, then Jill needs 3 hours less than Jack, so if Jack needs T hours, Jill will need T - 3 hours

Then we have the equation:

Y*(T - 3 hr) = 1 card

And we also know that when they work together, they need 2 hours, then:

(X + Y)*2 hr = 1 card.

So we have 3 equations:

X*T = 1 card

Y*(T - 3 hr) = 1 card

(X + Y)*2 hr = 1 card.

Now let's ignore the card part just for the equations, then we get:

X*T = 1

Y*(T - 3 hr) = 1

(X + Y)*2 hr = 1

To solve it, we first need to isolate one variable in one of the equations, let's isolate T in the first equation, we get:

T = 1/X

Now we can replace this in the second equation to get:

Y*((1/X) - 3hr) = 1

So now we have two equations:

(X + Y)*2 hr = 1

Y*((1/X) - 3hr) = 1

We need to do the same, isolate one variable in one of the equations, let's isolate Y in the above one:

Y*2hr = 1 - X*2hr

Y = (1 - X*2hr)/2hr

Now we can replace this in the other equation to get:

((1 - X)/2hr)*((1/X) - 3hr) = 1

Now we can solve this for X:

((1 - X)/2hr)*((1/X) - 3hr) = 1

(1 - X)*((1/X) - 3hr) = 2 hr

1/X - 3hr - X*2hr/X + X*2hr*3hr = 2hr

1/X - 3hr - 2hr + X*6hr^2 = 2hr

1/X + X*6hr^2 = 2hr + 3 hr + 2 hr = 7hr

1/X + X*6hr^2 = 7hr

Now we can multiply all by X to get:

X/X + X*(X*6hr^2) = 7hr*X

Now we have the quadratic equation:

(6hr^2)*X^2 - (7hr)*X + 1 = 0

We can find the solutions of this equation if we use the Bhaskara's formula, the solutions are:

$X = \frac{-(-7hr) +- \sqrt{(-7hr)^2 - 4*(6hr^2)*1} }{2*6hr^2} = \frac{7hr +- 5hr}{12 hr^2}$

Then we have two solutions:

X = (7hr + 5hr)/12hr = 1 hr^-1

And this is the rate at which Jack writes a card, so the actual units are:

X =  0.167 card/hr

Then Jill's rate will be:

Y = (1 - X*2hr)/2hr = (1 - 1card/hr*2hr)/2hr = -0.5 card/hr

So we have a negative rate, this makes no sense, so we can discard this option. The other solution for X is:

X = (7hr - 5hr)/12hr = 0.166... hr^-1

And this is the rate at which Jack writes a card, so the actual units are:

X =  0.166... card/hr

If we have this, then the rate for Jill will be:

Y = Y = (1 - X*2hr)/2hr = (1 - 0.167card/hr*2hr)/2hr = 0.333 card/hr

Then we have:

X = 0.167 card/hr

If we use the first equation, we get:

(0.166... card/hr)*T = 1 card

T = 1 card/((0.166... card/hr)) =  6 hours

Then Jack needs 6 hours to write a card.

And we know that Jill needs 3 hours less than that, then Jill needs 3 hours.