not sure how to do this, plz help

For return of 0%
z = (0-10%) / 20%
z = -0.5

For return of 5%
z = (5-10%) / 20%
z = -0.25

From the normal distribution table or following website:
http://www.statsoft.com/textbook/sttable.html#z


Probability between -0.5 and 0 is 0.1915, and
Probability between -0.25 and 0 is 0.0987

And so the probability between -0.25 and -0.5, i.e. the probability that the return will be between 0% and 5%
= 0.1915 – 0.0987
= 0.0928 (9.28%)

You can also check the results from following websites,
http://davidmlane.com/hyperstat/z_table.html
and input the figures:
mean = 10,
standard = 20,
choose between 0 and 5,
the shaded area is 0.092756, and so the above calculation is corretc.

you can ask your teacher to explain the homework.