Well, let's see. First we would need to find how much money he earns each DAY. To find THAT, we would need to multiply 760 times 1.5, which would be $1,140. Now, in order to find out how many DAYS it will take to make $3,420, you would divide 3,420 divided by 1,140, which will be 3. So it will take 3 days to earn $3,420.
I hope I helped! =D