The ratio of the amount of the oil bill for the month of February to the amount of the oil bill for the month of January was 5:4. If the oil bill for February had been $30 more, the corresponding ratio would have been 3:2. How much was the oil bill for January? Choose the correct choice from the following:
A) $ 60
B) $ 80
C) $ 100
D) $ 120
E) $ 140

Rationale: "5 : 4 = 15 : 12 and 3 : 2 = 18 : 12 . an increase in $ 30 increases the ratio by 3 : 12 . therefore an increase in $ 10 increases the ratio by 1 : 12 . therefore , january ' s bill was 12 ( $ 10 ) = $ 120 . the answer is d ."
Answer: D