<p>hi! on practice test #6, section 7, problem #10. i keep on doing it, and it seems easy, but i guess my brain is not working, because i can't get the right answer. for those of you who don't have the book, the question is:</p>
<p>10) 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 the two machines to make a total of 900 bolts?
a) 36
b) 72
c) 120
d) 144
e) 180</p>
<p>the answer is B, but i don't get it...please help! thanks in advance!</p>
<p>If you see "per min", "per hour", etc. - it's a rate question.</p>
<p>Just like with the distance formula
d = r * t
[mi] = [mi/hr] * [hr]</p>
<p>amount a
a = r * t
[bolts] = [bolts/hr] * [hr].</p>
<p>Just one difference with the "distance" questions: you use a combined rate.
r1 = 300 b/hr,
r2 = 450 b/hr,
both machines working together make
r = r1 + r2 = 750 b/hr.</p>
<p>Now back to the rate formula:
a = r * t
900** = 750[b/hr] * t[hr]
t = 900 / 750 = 1.2[hr]
1.2 hr is 1.2 * 60min. = 72min.</p>
<p>My purpose here is not to necro a 4 yr old thread, but to share a much faster solution method that leaves practically no room for error.</p>
<p>In one hour (60 minutes), 750 bolts will be made (450 + 300). This means that the answer will slightly greater than 60. The only answer remotely close to 60 is B, 72. You can verify the solution very quickly:</p>
.</p>