1. Since z=(470-500)/30=-1 then the desired proportion is (100-68.27)/2 which
is about 16%.

2. If z1=(520-500)/30=0.667 and z2=(560-500)/30=2 then the desired proportion
is (95.45-48.43)/2 which is about 24%.

3. Since z=(450-500)/30=-1.667 then the percentile rank is (100-90.11)/2 which 
is about 5%.

4. One way of solving this is to compute the proportion less than 554 seconds
for this sample. In this case z=(554-500)/30=1.8 and so the proportion is
92.81+(100-92.81)/2=96.4%. Since this proportion is more than 95% then the
performance of the DBMS is adequate.