Answer:
Ruben ate 6 pieces out of 8.
Explanation:
We will understand 3/4 first. If we divide a pie into 4 pieces then Ruben ate 3 pieces of pie.
Further If we divide these 4 and 3 pieces to 8 and 6 then Ruben will eat 6 pieces out of 8.
Now we will show this mathematically

Ruben ate 6 pieces out of 8.