Hi i need to find the efficeiny of a project
i have 60 minutes availalble
i have a target of 78
if i have no stoppages and produce 78 i am 100%
if i have 30 mins stoppages and produce 39 i am 100%
if i have no stoppages and produce 39 i am 50%
can anyone tell me an easy wy of calculating this

thanks
kevin