Question

A light bulb consumes 600 watt-hours per day. How long does it take to consume 2100 watt-hours?

Answer 1

Units are your best friend here. You can make equations out of stuff with equal units, so you have somethings:

Something in terms of watt-hours, another thing in watt-hours per day, and you want to find how much time something takes. So if we write Watt as W and Hours as h and Days as D we can see:

600 W*h/d = 600 Watt-hours per day
2100 W*h = 2100 Watt-hours
??? time, so probably in days, so we can make an equation out of this stuff. Try piecing it together and see if you can make an equation with a variable.