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I am using cross-multiplication here, but it is definitely giving me the wrong answer. Here is the question:

If 1200 bits are transmitted in 1 second, how many milliseconds does it take to transmit 3 bits?

1000ms = 1s
1200b = 1 s
3b = x

I use x to represent what we need to find.

Here is how I set up the formula:

1200 = 1
/3     /x
1200x = 3

1200x = 3
x = 3/1200
x = .0025s

.0025s * 1000s = 2.5 milliseconds

Answer seems off. Any idea what I am doing wrong?

JohnMerlino
  • 621

2 Answers2

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Your answer is correct. What leads you to the opposite assumption?

wonce
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You have the right answer, but you've arrived at it incorrectly. Specifically, your equality is set up incorrectly. The given that you have is that 1200 bits are transmitted in a second. The "in" in this case should translate to "per" (think 'miles per hour'), which should trigger in your mind the proportion 1200/1. Then, you are asked to find the time to transmit 3 bits. Using the same format, this would translate into 3/x (3 bits per x seconds). Then, because the transmission rate is constant, you know that these two proportions yield the same value, so you can set them to be equal to each other and then solve.

1200   3
---- = -
  1    x

You've swapped the 3 and the 1, which means you've got both bit factors on one side and both time factors on the other, which isn't the way proportions are set up. In this case, it doesn't affect the answer because 3 * 1 = 1 * 3, but in future, keep in mind that you need to keep your units (in this case "bits per second") the same on both sides of the equation in order to accurately use cross-multiplication to solve.

It'a also pretty easy to check this answer, since we can note that 1200 bits per 1000 ms means a transfer rate of 1.2 bits per millisecond. So:

1ms = 1.2b
2ms = 2.4b
2.5ms = 2.4b + (1.2)/2b = 2.4b + 0.6b = 3b
Roger
  • 201