ANSWER
Each yard cost $6.00
Step-by-step explanation
If 3 1/2 yards cost $21.00, we have to divide the total cost by the number of yards bought to find the cost of 1 yard.
To do the division first we have to change the mixed number into an improper fraction:

Now do the division with the KCF rule:
Keep the first fraction
Change the division sign into multiplication sing
Flip the second fraction
Here the first fraction is a whole number, it means that the denominator is 1:

Now we can simplify this fraction, because 42 is a multiple of 7. So, dividing both numerator and denominator by 7 we have:
