There are hundreds of units we encounter in math problems. We sometimes get confused on how to deal with them. But there is only one thing that we need to remember when dealing with them, that is to always take note of the units we are using and we are to convert.
Conversion of units are not that simple as abc. That is, if we cannot identify what conversion factor to use, we will not be able to solve them.
First to take note of conversion of units is whether the units are of the same physical quantities or quantities in general. Say we have meter, which is a unit of length, to be converted to cubic meter, a unit of volume. We are dealing with two different quantities at the same time. Thus, we cannot convert meter to cubic meter because how can we convert it when they are of different quantities? We should always remember that measurements that can only be converted to another unit should be of the same physical quantity. So in the example, we can convert meter to feet, inches, yards, miles, kilometer and etc.
In the problem, we are to find how much should Sarah walk in "feet" if she has already walked 64200 yards.
All we just have to do is subtract how much she have walked from how much she should have walked. But since all given have different units, we need to convert each one of them into feet to have the same unit.
Recall that there are 5,280 feet in a mile.
So we first convert hiw much in feet should she walk.
42 miles x (5280 feet / 1 mile) = 221,760 feet
Then we solve for how much have she already walked. Recall that there are 3 feet in a yard.
64200 yards x (3 feet / 1 yard )= 192,600 feet
Since we already have the same unit, we can now easily subtract the values to get how much she will still be walking.
221,760 feet - 192,600 feet = 29,160 feet
Therefore, Sarah still needs to walk 29,160 feet to finish her course.