数学题:一艘船在静水中每小时行驶40千米，水流的速度是5千米/小时。这艘船从甲地顺流而下，7小时到达乙地。如果该船从乙地逆流而上，需要几小时到达甲地?

数学题解答：顺流而下，实际的行驶速度是船的速度+水流的速度，先根据这一点算出两点的距离：

(40 + 5) × 7 = 315（千米）

逆流而上，实际速度则是船速减去水流速度，需要的时间是：

315 ÷ (40 – 5) = 9（小时）

推荐阅读：
5 Skills All Successful Content Writers Have Mastered　 Blogging for Dummies in 2019: The Only Cheat Sheet You Need　 Are Readers Reaching Your Call To Action?　 How to Manage a Content Marketing Plan for Your Blog　 4 Simple Techniques To Bring Your Dead Blog Back To Life　 How to Earn Bitcoin Through Blogging　 5 PayPal Alternatives for Bloggers and Solopreneurs　 Can WordPress Make Other E-commerce Platforms Obsolete?　 How to Start a Food Blog (A Step-By-Step 2019 Guide)　 3 Possible Solutions to the Most Common Corporate Blogging Probl