Xiaodong’s father drove him home at the school gate at 4:00 every afternoon. This afternoon the teachers had a meeting, and the school ended at 3:00. Xiaodong decided to walk first. Arrived home 30 minutes earlier, how many minutes did Xiaodong walk.
小东的爸爸每天下午4：00开车在校门口接他回家，今天下午教师开会，学校3：00放学，小东决定自己先步行，途中正好与开车来接的爸爸碰到，结果爸爸今天早30分钟到家，小东步行了多少分钟．
<result>45</result>