What standard score represents 1.5 and 2 standard deviations below mean?

Look it up in the z table. Do you have a stats book? The table is surely in there.

Or, you can do it the hard way:

\frac{1}{\sqrt{2\pi}}\int_{-\infty}^{z}e^{\frac{-t^{2}}{2}}dt:):)


If you do not have a table, remember the Empirical Rule says that 34% of the data falls 1 SD below the mean.

Here's the normal graph: You can easily approximate it from looking at the graph.