If the sun's mass is about average, how many stars are there in the milky way galaxy? the mass of the sun is of the order of 1030 kg and the mass of the milky way galaxy is of the order of 1042 kg.

The answer is:

the sun has the average mass of all the stars, then divide the mass of the galaxy by the mass of the sun.  

Specifically, she wants you to do this division by subtracting exponents. 10^30 has 30 zeroes after it. 10^42 has 42 zeroes. That's a difference of 12 zeroes, which is a trillion stars.  

That's approximately right, but since the sun is not exactly the average size, the real answer is about 1/3 that, 300 billion