Machines A and B always operate independently and at their respective constant rates. When working alone, Machine A can fill a production lot in 5 hours, and Machine B can fill the same lot in x hours. When the two machines operate simultaneously to fill the production lot, it takes them 2 hours to complete the job.

What is the value of x ?

3~$1 \over 3$~

3

2~$1 \over 2$~

2~$1 \over 3$~

1~$1 \over 2$~