You would multiply the cups of flour by 3 and the cups of water by 3. which would give you 9 cups of flour and 6 cups of water. the ratio is 1.5 cups of flour to 1 cup of water.
The interval corresponds to the part of the distribution lying within 2 standard deviations of the mean (since 500-2*100=300 and 500+2*100=700). The empirical rule states that approximately 95% of the distribution is expected to fall in this range.