6 hours alone 4 hours both 2nd person how many hours

Question

Batman can clean up all the crime in Gotham City in 6 hours working alone. If Robin helps, the job takes only 4 hours. How long would it take Robin to clean up all the crime but himself

Answer 1

r represents the hours Robin needs

 

4/6 + 4/r = 1

2/3 + 4/r = 1

 

Can you solve for r and answer?