Solve the following problem. Three teams are hired to dig a well. If every team would forced to do it alone, they would spend: first - 30 days, second - 40 days, third - 20 days. But then contract received mixed team contained from 1/2 workers from first team, 1/3 - from second team and 1/4 from third team (all workers are equally productive within team). How many days mixed team will spend on digging well?