A certain business printer can print 40 characters per second, which

Rate at which business printer prints = 40 characters /second
Rate at which average printer prints = 10 characters /second
Rate at which electric typewriter prints = 2 characters /second which equates to 2*60 = 120 characters /minute
correct answer - D

Shortcut
Find the first rate in minutes.

"Certain" business printer's rate in characters, \(c\), per 1 minute:

\(\frac{40c}{1sec}*\frac{60}{60}=\frac{2,400c}{60secs}=\frac{2,400c}{1 minute}\)

Shortcut:
All rates are per 1 minute
Let the different machines be B, AP, and ET
Rate of B is 5 times as fast as that of AP
Rate of AP is 4 times as fast as that of ET

Hence B's rate is (5*4) = 20 times as fast as ET's rate

\(2,400c=20*ET\)
\(ET=\frac{2,400c}{20}=120c\)
\(ET=\frac{120c}{1 min}\)

In one minute, the electric typewriter can type 120 characters

Answer D

In steps
"Certain" business printer's rate from above:

\(\frac{2,400c}{1 minute}\)

That rate is 4 times as fast as the average printer's rate, AP:

\(2,400c=(4*AP)\)
\(AP=\frac{2,400c}{4}=600c\)
\(AP=\frac{600c}{1 min}\)

THAT per minute rate, AP, is 5 times as fast as an electric typewriter's per minute rate, ET:

\(600c=5*ET\)
\(ET=\frac{600c}{5}=120c\)
Electric typewriter rate: \(\frac{120c}{1min}\)

In one minute, the electric typewriter can type 120 characters

Answer D

*All rates based on a prior PER MINUTE rate are also per minute

Since the rate of a business printer is 40 characters per second, then the rate of an average printer is 10 characters per second, and the rate of an electric typewriter is 2 characters per second. Therefore, in one minute, the electric typewriter can print 2 x 60 = 120 characters.

Answer: D

Business printer: 40 characters per second

Average printer: \(\frac{40}{4}\) =10 characters per second [business printer is 4 times as fast as an average printer]

Electric typewriter: \(\frac{10}{5}\) = 2 characters per second [average printer can print 5 times as fast as an electric typewriter]

Characters per minute can an electric typewriter print: 1 sec = 2 characters => 1 * 60 = 60 * 2 = 120 [1 minute has 60 seconds]

Answer D

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