It takes Ralph 5 hours to paint a fence. Lisa can do the same job in 12 hours. If Ralph paints alone for 30 minutes before Lisa begins helping, how long must they work together to finish painting the fence?

My Solution:

Let x = the time it takes both to paint the fence

I do not know where 30 minutes should be in the equation.

1/5 + 1/12 = 1/x

If this is the wrong equation, why is it wrong?