Use the information in the 1st question to you out in questions 2 and 3

Problem One
There is nothing wrong with the answer you got.

Gives and required.
1 batch of cookies requires 1/3 cup of sugar
x batches of cookies will need 5 cups of sugar.

Proportion
1 / x = 1/3 / 5

Solve
You could change 1/3 into 0.33333333
1/x= 0.33333333/5     Cross multiply
5 * 1 = 0.333333x       
5 = 0.33333333x       Divide by 0.3333333
x = 5/0.33333333
x = 15 <<<<< Answer 

Problem 2
Givens
9 cups of sugar

Proportion
1 batch requires 1/3 cup of sugar
x batches will require 9 cups of sugar.

1/x = 1/3 //9
1/x = 0.3333333 / 9    Cross multiply
9*1 = 0.3333333 x
9 = 0.33333333x   Divide by 0.33333333
9/0.33333333 = x 
x = 27 batches of cookies. <<< Answer 

Ignore the very small decimal you get. It does not matter. It is a rounding error.

Problem 3
Let the whole shift = x
Let the shift Fred signed up for = x/2

Now x/2 has to be split 3 ways.  How do we do that?

Well we can represent it.



The best way to proceed is to put a 1 under the three.



Now you get to invert the bottom fraction (3/1) so that it becomes (1/3) and multiply the two fractions together.



Each friend will work 1/6 of a whole shift.