One inlet pipe fills an empty tank in 5 hours. A second inlet pipe fills the same tank in 3 hours. If both pipes are used together, how long will it take to fill ~$2 \over 3$~ of the tank?

~$8 \over 15$~ hr

~$3 \over 4$~ hr

~$5 \over 4$~ hr

~$15 \over 8$~ hr

~$8 \over 3$~ hr