ED:8cm,DC:10 so the air is HD:18
the possibilities are probably b, using the elimination method
Do the exponents first. 10 to the -4 is 1/10000 or 0.0001 and 10^ -5 is 0.00001. You end up with (1.7*0.0001)(5*0.00001) which is 0.00017*0.00005 which is 0.0000000085 or 0.085*10^-8
answer: you're missing the problem
81-1= 80
80 / 2 = 40
D = 40
So:
40(2) = 81 - 1