Question

40 ounces is how many pounds?

Answer 1

2.5 pounds

Answer 2

It's helpful to write down a series of steps to methodically arrive at an answer. What you can do is \[(40 oz)* ({0.0635 lb \over 1 oz})\] and notice that the "oz" units mathematically cancel out and you're left with units of lb. This is called dimensional analysis and if you know relations between two units you can do so with ease like in the example below:

6 ft to meters.\[(6ft)*({12 inches \over 1 ft})({2.54 cm \over 1 inches})({1 m \over 100 cm})\]. You can see that the units cancel out and that you're left with meters. The thing you have to know is that 12 in/ 1ft is equal to one (not mathematically) in terms of these dimensions because they represent the same thing. What this implies is that you can invert the fraction to (1ft/12ft) and it's still one. So this means you only need one relation for a set of units to use them in conversions. Mastering this will make your life easier later.