Part A) The answer is D) 0.3m = 12.
Part B) The answer is 40 minutes.
If m is the total number of minutes it takes him to complete his homework, 30% of m is given by 0.3m. Since 30% was done in 12 minutes, we have 0.3m=12.
To solve this, we divide both sides by 0.3:
0.3m/0.3 = 12/0.3
m = 40