How do WhatsApp, Facebook, Google maps, etc.work?

A can do of a work in 10 days. B can do of the work in 20 days. In how many days can both A and B together do the work?

A can do a certain work in 15 days, while working together, A and B can do the same work in 7.5 days. How many days will B alone take to do the same work?

A can do a piece of work in 9 days and B can do the same in 12 days. In how days can the work be completed if A and B work together?

If A can do a piece of work in 4 days, B can do the same work in 8 days and C can do the same work in 12 days , then working together , how many days will they take to complete the work ?