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
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 $120.
<result>D</result>
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?