



 
Hi Dave, I constructed a table to describe the setup. The body of the table is 2 by 2 with the two rows representing the colours, red and green and the columns representing the shapes, cubes or balls. In the margins are row and column sums and in the lower right corner is total number of objects which is 80. I filled in the numbers you know.
Since there are 30 red objects and 20 of them are cubes the remainder, 30  20 = 10 must be balls.
In a similar way you can fill in the remainder of the table. Once the table is complete you can use it to calculate the probabilities. Let us know if you have difficulty completing the problem. Harley  


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