Balls in Boxes Probability I am quite stuck here as to how my book achieves an answer.
The question reads that there are two boxes, lets call them $A$ and $B.$
Box $A$ has $24$ balls; $3$ red, $8$ yellow, $13$ green.
Box $B$ has $18$ balls; $5$ red, $7$ yellow, $6$ green.
the question ask: what is the probability of choosing one ball from each box, such that both balls are the same color$?$
Once again I understand that P(event)= outcomes of event/ total outcomes 
now the book states the answer as being $149/432$, but I do not know how they did so.
My attempt:
I know that my sample space is $432$, because there is an intersection from box $A$ and $B$; such that $24\times 18$ yields the total possible outcomes. So I got $432$ part. 
Then if I pick a red from box $A$ then I have a $5/18$ chance picking red from $B$
if I picked a yellow from box $A$ then I have a $7/18$ chance of picking yellow from $B$
and if I picked a green from box $A$ then I have a $6/18$ chance of picking a green. But I don't know how to use this information to my advantage in order to solve it

Imagine for a moment that the balls are not only colored, but numbered as well.
Let $A_r,B_r,A_y,B_y,A_g,B_g$ represent the events that you select a red from box $A$, red from box $B$, yellow from box $A$, etc...
Let $E$ be the event that you select the same color ball from each box.
Note that $E=(A_r\cap B_r)\cup (A_y\cap B_y)\cup (A_g\cap B_g)$ and that each of these intersections are disjoint events.
This implies that $|E|=|A_r\cap B_r|+|A_y\cap B_y|+|A_g\cap B_g|$
Let us turn our attention now to trying to calculate $|A_r\cap B_r|$.  How many ways can we choose a red ball from box $A$ and a red ball from box $B$?


*

*Pick which red ball it is from box $A$ (remember, we are imagining that they are numbered to assist in the calculations).  There are $3$ choices.

*Pick which red ball it is from box $B$.  There are $5$ choices.


There are then $3\cdot 5=15$ total ways of picking a red ball from both boxes.
Similarly, there will be $8\cdot 7=56$ ways of picking a yellow from each and $13\cdot 6=78$ ways of picking a green from each.
We get then that $|E|=15+56+78=149$
Remembering that our sample space is all ways of picking a ball from each and is of size $24\cdot 18=432$ we have a final answer of:
$$Pr(E)=\frac{149}{432}$$
