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Laila12618
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Laila12618
Oven A takes 5 hrs longer than oven B to bake y pizzas. If oven A + oven B can bake 3y/2 pizzas together in 2 hrs, approximately how long (in minutes) does it take to oven B to bake y pizzas?

(A) 80
(B) 90
(C) 100
(D) 110
(E) cannot be determined

Posted from my mobile device

Given

Oven A + Oven B together can bake 3y/2 pizzas in 2 hours

Time taken to bake y pizzas = \(\frac{2 * 2 }{ 3}\) hours = \(\frac{4}{3}\) hours

Assume :

Time Oven B takes to bake y pizzas = t hours

Time Oven A takes to bake y pizzas = t+5 hours

Time taken when both Oven A and Oven B works together to bake y pizzas = \(\frac{t(t+5)}{t + t + 5}\)

\(\frac{t(t+5)}{2t + 5} = \frac{4}{3}\)

\(3t^2 + 7t -20 = 0\)

\(3t^2 + 12t - 5t - 20 = 0\)

(t+4)(3t-5) = 0

t = \(\frac{5}{3}\) hours

Time in minutes = \(\frac{5}{3} * 60 = 100\) mins

Option C


I would like to understand better where this \(\frac{t(t+5)}{t + t + 5}\) comes from.

I understand t and t+5, but I don't understand how and why the fraction is built.

Could you please clarify?
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­Here are two additional ways to do this problem:

Method 1 (based on time):
A = x + 5 hrs for y pizzas
B = x for y pizzas

So for 3y/2 pizzas:
A = 3(x+5)/2
B = 3x/2

Total time then: 2/(3x+15) + 2/3x = 1/2 (Since 2 hours when they work together).

Solving for this gets the answer.

Method 2 (based on work):
A = x + 5 hrs for y pizzas
B = x for y pizzas

Time taken to work together: 2 hours
2/x + 2/(x+5) = 3/2

Solving for this gets the answer.
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Time taken to bake y pizzas = 4.3 hrs > please explain this calculation and process in detail.
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shubhamagarwal44
Time taken to bake y pizzas = 4.3 hrs > please explain this calculation and process in detail.
I'm assuming you meant to ask 4/3 hrs viz. time taken by A and B working together to bake y pizzas.

Since we are given that, oven A and B working together, time taken to bake \(\frac{3y}{2}\) pizzas is \(2\) hours, and we know that Rate * Time = Work

The Rate of A and B working together will be => Work / Time = \((3y/2)/2\) = \(\frac{3y}{4}\)

Now, time taken to bake y pizzas by oven A and B together will be => Work / Rate = \(\frac{y}{(3y/4)}\) = \(\frac{4}{3}\) hrs

Well, above calc. is not really required, if you approach it directly calculating (Rate of A + Rate of B) * Time taken working together = Work done working together

\((\frac{y}{t+5} + \frac{y}{t})*2 = \frac{3y}{2}\)

And get time taken by B for y pizzas, i.e. t = 5/3 hrs => 100 mins.

Hope it helps
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