How to find a common denominator of 6/7 and 5/13

Just invert it like:


There you have your common denominator! Don't forget to multiply the numerator as well.

The probability would be 0.1193.

We compute the z-score for each end of this interval.  For the lower end, 66 inches, 

z = (X-μ)/σ = (66-68)/3 = -2/3 = -0.67

For the upper end, 67 inches, 

z = (X-μ)/σ = (67-68)/3 = -1/3 = -0.33

Using a z-table (http://www.z-table.com) we find the area to the left of (lower than) the lower limit is 0.2514.  The area to the left of (lower than) the upper limit is 0.3707.  To find the area between these, we subtract:

0.3707 - 0.2514 = 0.1193.