Dividing by 1/2 means multiplying by 2. So 30 divided by 1/2 = 30 x 2 = 60. Then 60 + 10 = 70. The common mistake is dividing 30 by 2, which would give 15, but the question specifies dividing by 1/2, not 2. Options C (15) and B (35) result from this error.