Seriously i dont get this qs

[candygal79]

A metal company's old machine makes bolts a constant rate of 100 bolts per hour. The company's new machine makes bolts constant rate of 150 bolts per hour. If both machine start at the same time and continue making bolts simultaneously, how many MINUTES will it take the two machines to make a total of 300 bolts ?

A 36

B 72

C 120

D 144

E 180

[GMATGuruNY]

A metal company's old machine makes bolts a constant rate of 100 bolts per hour. The company's new machine makes bolts constant rate of 150 bolts per hour. If both machine start at the same time and continue making bolts simultaneously, how many MINUTES will it take the two machines to make a total of 300 bolts ?

A 36

B 72

C 120

D 144

E 180

Rates are ADDITIVE.
When the two machines work together, their combined rate = 100+150 = 250 bolts per hour.
Time for the two machines to produce 300 bolts = w/r = 300/250 = 6/5 hours.
Since 1 hour = 60 minutes, we multiply by 60 to convert hours to minutes:
(6/5) * 60 = 72 minutes.

The correct answer is B.

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Hi candygal79,

The answer choices to this question provide a fantastic shortcut.

With the two machines that we're given, we know that...

Old Machine = 100 bolts/hour
New Machine = 150 bolts/hour

If they start at the same time, the two machines will create 250 bolts/hour

The question asks how long it would take the two machines to make 300 bolts so we know it's going to be more than 1 hour.

If the two machines worked for 2 hours, then they'd make 500 bolts, but that's too many bolts.

We need an answer that's more than 1 hour, but less than 2 hours. The question also asks for an answer IN MINUTES. The ONLY answer that fits is the one that's between 60 minutes and 120 minutes.

Final Answer: B

GMAT assassins aren't born, they're made,
Rich

[Scott@TargetTestPrep]

A metal company's old machine makes bolts a constant rate of 100 bolts per hour. The company's new machine makes bolts constant rate of 150 bolts per hour. If both machine start at the same time and continue making bolts simultaneously, how many MINUTES will it take the two machines to make a total of 300 bolts ?

A 36

B 72

C 120

D 144

E 180

We can let t = the number of hours to produce 300 bolts and create the equation:

(100 + 150) x t = 300

t = 300/250 = 6/5 hours, which is 72 minutes.

Alternate Solution:

We note that the two machines working together will produce 100 + 150 = 250 bolts in 60 minutes. Also, we notice that in 120 minutes, the two machines will produce 250 x 2 = 500 bolts. Since 300 is between 250 and 500, the time necessary for the two machines to produce 300 bolts must be between 60 and 120 minutes; the only possible choice is 72 minutes.

Answer: B