Answer:
Donald needs 3 days to earn 3420 dollars
Explanation:
Donald sells 760 marbles everyday. He sells each marble for 1.5 dollars
amount collected in one day=

In one day the amount earned is 1140 dollars
we need to find the number of days needs to earn 3420 dollars
Number of days needed is

So its around 3 days
Donald needs 3 days to earn 3420 dollars