You have to start with thy x and the division than the minus and the plus
I think its is 4/5 but not positive
Out of 15 marbles, 3 are yellow, which leaves 12 more marbles that are NOT yellow. 12/15 would be your answer
hope this helps
400 : 1kg
400 : 1000
1 : 2.5
n = 2.5