Lets say I want to convert from Feet to Inches:
1 Ft= 12in.
I want to know how many inches are in 5 feet.
I start with what I know: 5ft
I then multiply this by a fraction that consists of the same unit for the denominator and the unit I want to convert to for the numerator:
5ft (12in/1ft) . Notice how I only used 1ft for the denominator, make sure you do this every time as it makes things easier. I also do this because I know for every 1ft there are 12in.
After this you cancel units and every time you have units that are "diagonal" from each other you can cross them out.
For example: 12ft (12in/1ft) = 12 (12in/1) ; this helps cancel units that have already been converted throughout the math and helps you keep track of the units of your final answer.
Another example: 12ft (3ft/2seconds) ; for this example you cannot cancel out the unit of ft for the 12 and the seconds for the 2 because they are not the same unit. You need to make sure they are the same units. This answer would just be 12*3=36ft . 36ft/2seconds = 18ft/sec
For the original problem we have: 5ft
5ft(12in/1ft) .... 5(12in/1) = 60 in. , our final answer is in inches because that is the only unit we have left. Therefore there are 60 inches in 5 feet.