We need to work out the z score. We know that 96 and 144 are on either side of the mean (120 mm) by 24 mm. The standard deviation is 12 mm so the range 96 to 144 is 2 standard deviations from the mean. This means we look up 2 in a normal distribution table. The percentage corresponding to z=2 is 97.72% or the rule-of-thumb value of about 98%.