Let's look at an example:
2.24 x 21.6

Do normal multiplication and get
48384 This is not correct, until we put the decimals in. To know how to do this, count the number of decimal spaces from the factors. In 2.24, there are 2, because .24. In 21.6 there are 1 because of .6. This mean three places, so 48384 becomes
48.384
~theLocoCoco
Fit Fast: a set feet per class => y = Ax
Stepping Up: a monthly fee plus an additioal fee per class => h = Bx + C
You can discard the second and the fourth systems because they do not have the form established from the statement.
The first system produce an obvious result given that is represents an option that is always better than the other 5.5x will be lower than 7.5x + 10 for any positive value of x, and so there is no need to make any comparission.
The third system is
y = 7.5x and y = 5.5x + 10 which need to be solved to determine when one rate is more convenient than the other.
Answer: y = 7.5x and y = 5..5x + 10
3 [ (20-4) / 2 ]= 3 * 16/2= 3 x 8 =24
5,(9)=5 9/9= 54/9=6
6/8/4= 6/2=3