求一道DS数学题

rushlin

For each day at sea a sailor is paid a fixed sum. For each day aheadof the scheduled arrival time, he gets twice the regular daily earnings. If thetotal time at sea was scheduled for 20 days and a sailor earned a total of3,850 dollars for the entire trip, how many days ahead of schedule did the shiparrive?

1. If the ship had arrived one day ahead of the schedule, the sailorwould have earned 4,200 dollars.
2. The sailor's regular daily earnings were $175.
选D

rushlin

（1）不是很懂什么意思

sally20130106

设x天提前到达，设daily regular earning为y， 那么：
由已知可得方程：
（20-x)y+2xy=3850
然后由（1）得：
x=1时，那么有
19y+2y=4200
两个方程，两个未知数，可以解得x=-0.75， 说明他晚回来了0.75天
由（2）直接可以将y=175代入第一个方程，可以解的x=2
选D