Divide the traveled distance over the time it takes to travel it to find the speed as a unit rate. Then, compare with other unit rates to find if they are equivalent.
[tex]\frac{16\text{ miles}}{4\text{ hours}}=4\text{ miles per hour}[/tex]A)
[tex]\frac{24\text{ miles}}{6\text{ hours}}=4\text{ miles per hour}[/tex]B)
[tex]\frac{20\text{ miles}}{6\text{ hours}}=3.333\ldots\text{ miles per hour}[/tex]C)
[tex]\frac{12\text{ miles}}{4\text{ hours}}=3\text{ miles per hour}[/tex]D)
[tex]\frac{4\text{ miles}}{1\text{ hour}}=4\text{ miles per hour}[/tex]Notice that the ratios given in the options A and D are equivalent, but those in B and C are not.