<p>The Smith Metal Company's old machine makes 300 bolts per hour. Its new machine makes 450 bolts per hour. If both machines begin running at the same time, how many MINUTES will it take for the two of them to make a total of 900 bolts?</p>
<p>a.36
b.72
c.120
d.144
e.180</p>
<p>Correct=B. Why? Even the answer explanation wasn't clear to me. Help?</p>
<p>There is an algebraic solution to this, but an easier way to look at it would be to look at the answer choices. since they make 750 bolts per 60 minutes (hour), you can cross off c d and e since you would make more than 1500 bolts in those minutes. And you can cross off a because you know that you need more than 60 minutes since the machines make 750 bolts in 60 minutes and a is less than one hour. That leaves you with b!</p>
<p>The number of bolts that the old machine makes in in x minutes is 5x (because 300 hundred per hour is 5 per minute).</p>
<p>The number of bolts the new machine makes in x minutes is 7.5x (because 450 per hour is 7.5 per minute).</p>
<p>The number of bolts that the two machines working together make in x minutes is 5x + 7.5x = 12.5 x.</p>
<p>So we want to know what x is – i.e. 12.5 x = 900. Solve for x.</p>