Hello, you can simply use this formula:
(This formula can be used in any type of problems similar to this one)
a x b / a + b
Let a = Trevon
Let b = Cody
10 x 7 / 10 + 7 = 70/17 = 4.11~
For the latter, however, if you are not given all the variables, you can simply plug in the values they gave you.
Working together, Jenny and Natalie can mop a warehouse in 5.14 hours (In this case, you would set the whole equation equal to 5.14, since it's how long it would take if they worked together.) Had she done it alone, it would have taken Natalie 12 Hours. How long will it take Jenny to do it alone?
Let a = Jenny
Let b= Natalie
And.... finish up from there by using the formula