Answer:
60 red marbles
Explanation:
If John picks 10 marbles and 6 of them are red, there is a 60% chance of pulling out a red, which also means 60% of the total number of marbles are red.
(To be fair, 10 marbles is not a lot to base an estimate on, but I digress.)
Therefore, if there is a total of 100 marbles in the bag, 60% of 100 is 60. Our estimate will be 60 red marbles.