First, you write: boat speed in still water as *x*, current speed as *y*. When going upstream, speed is *x - y*; downstream, *x + y*. Use given distances and times to make two equations, then add or subtract them to eliminate one variable—like cancelling *y* by adding the two equations. Understand?
Every morning, I measure water for my betel-vine with the same care—too much or too little, and the leaves suffer. It's all about balancing two forces, just like this *x* and *y*.
But, aiee, I must tell you—in real life, like my crumbling IAS hopes, sometimes the current of time is stronger than any boat. You can solve the equation, but will you reach the shore? That part, the book doesn't say.
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