This is simply a question about ratios. This ratio means that for every 1 cm on the map, there are 12 km in actuality. You can set this problem up in fractions:
1 cm 12 cm
-------- = ----------
12 km x
To solve for x, we cross-multiply, for the moment ignoring units. Remember, your answer will be in km.
1*x=12*12
x=144
If x=144, then for 12 cm, there are 144 km.
You can use elimination to solve systems of equations with 3 equations. I know how to solve systems of equatons with 3 equations, but I use a different process, I don't know how to use the elimination method.
One factor of 12 is 2. The other factors of 12 are 3, 4, and 6.
Approximately 68% of a normal distribution lies within one standard deviation of the mean, so this corresponds to students with scores between

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Because there is no whole number that you can divide into both the numerator and the denominator to get a proper fraction.