A pipe is clogged so that only 2 ounces of cleaner can run through the pipe per minute. After fifteen minutes, the cleaner has unclogged it enough that 3 ounces can run through per minute. Ten minutes later, the clog is cleared enough for 4 ounces to run through per minute. How many ounces of cleaner were used after 30 minutes? Let's think first. Chain of thought:
For 15 minutes at 2 ounces per minute, 2 * 15 = 30 ounces were used. For the next 10 minutes at 3 ounces per minute, 3 * 10 = 30 ounces were used. There are 30 - 15 - 10 = 5 minutes remaining. For the next 5 minutes at 4 ounces per minute, 4 * 5 = 20 ounces were used. After 30 minutes, 30 + 30 + 20 = 80 ounces of cleaner were used.
Therefore, the answer is 80.