A rides a bicycle 100 kilometers every day, B rides a bicycle 70 kilometers on the first day, and every day thereafter travels 3 kilometers more than the previous day. The two set off at the same time, how many days will B catch up with A?
甲骑车每天行100千米，乙骑车第一天行70千米，以后每天都比前一天多行3千米．两人同时出发，第几天乙追上甲？
<result>21</result>