Also, pro-tip if you want your math professors to hate you: If you want to get something done quickly, do it as follows:
Round every number to the nearest power of 10, so your numbers are just 10^x and 10^y respectively. Also make sure X > Y.
For addition: The result is 10^x.
For subtraction: The result is 10^x or -10^x..
For multiplication: The result is 10^(x+y)
For division: The result is 10^(x-y)
For power of 2: The result is 10^(2x)
For square root: The result is 10^(x/2)