To solve this we'll put the question in this format: x% of y things = z things
We have the percent so we'll put in 60 for x.
Now the total number of balls will be considered y since we have the number of red balls but not blue ones. so we end up having..
60% of y balls = 720 balls
Of is another word for multiplication, therefore:
60/100*y=720
Solve for y
y=1200
So we have 1200 balls