Ken is working this summer as part of a crew on a farm. He earned $\$8$ per hour for the first 10 hours he worked this week. Because of his performance, his crew leader raised his salary to $\$10$ per hour for the rest of the week. Ken saves 90% of his earnings from each week. What is the least number of hours he must work the rest of the week to save at least $\$270$ for the week?