Have you really lost your marbles?

Logic

I have four boxes, each containing a number of red marbles and blue marbles.

	Box A	Box B	Box C	Box D
$\text{Red marbles}$	$70$	$y$	$2$	$7$
$\text{Blue marbles}$	$30$	$3$	$98$	$53$

If the probability of randomly selecting a red marble from Box A is $a$ , and the probability of randomly selecting a red marble from Box B is $b$ , then $a < b$ .

Suppose we group all the marbles in Box A and Box C into another Box AC; likewise we group all the the marbles in Box B and Box D into another Box BD. Now, there is a higher probability of randomly selecting a red marble from Box AC than from Box BD.

What is the sum of the smallest and the largest possible values of $y$ for which the above criteria is satisfied?

Have you really lost your marbles?

Image Credit:Flickr Lyle.