Maomao rides on the back of an ox to cross the river. He has 4 cows A, B, C, and D. It takes 20 minutes for A to cross the river, 30 minutes for B to cross the river, 40 minutes for C to cross the river, and 50 minutes for D to cross the river. Maomao can only drive 2 cows across the river at a time, but how many minutes will it take to drive 4 cows to the other side?
毛毛骑在牛背上过河，他共有甲、乙、丙、丁4头牛，甲过河要20分钟，乙过河要30分钟，丙过河要40分钟，丁过河要50分钟．毛毛每次只能赶2头牛过河，要把4头牛都赶到对岸去，最少要多少分钟多少
<gadget id="calculator">50 + 80 + 30</gadget>
<output>160</output>

<result>160</result>