F40% of the mixture is ryegrass, what is the ratio of ryegrass to bluegrass

The correct answer is:  "2/3" ; or write as:  "2:3" .

This is asumming that the mixture consists entirely of ryegrass and bluegrass and "nothing else".

Explanation:

If 40% of the mixture is ryegrass; and the rest of the mixture is bluegrass; 

Then the entire mixture is 100% ;

and the bluegrass is:  100%  − 40% = 60%.

There would be 40% ryegrass to 60% bluegrass:

40% / 60% = 4/6 = (4÷2) / (6÷2) = "2/3" ; or write as:  "2:3" .