The time it takes for babies to learn to walk is known to be normally distributed with mean 9 months and standard deviation of 1.5 months. What percent of babies learn to walk?

 99.7% of babies learn to walk within 3 standard deviations from the mean in a normal distribution.

Explanation:   In the normal distribution given here, that breaks down to about 68.2% between 7.5 and 10.5 months, 13.6% between 6 and 7.5 months, 13.6% between 10.5 and 12 months, 2.1% between 4.5 and 6 months and 2.1% between 12 and 13.5 months.

This is applying the percentages for the Standard Deviations shown on a Gaussian Curve.