Ruma and Rohit can complete a work in $20$ days, Rohit and Sobha can complete that work in $15$ days. Ruma and Sobha can complete that work in $20$ days. Let's calculate how many days they will complete the work together. 

Mary and David can do a work individually in $10 \text{days}, 15 \text{days}$ respectively. At first mary alone worked for  $4$ days, then David alone worked for $5$ days and left. Maria came and completed the remaining work in $4$ days. If Mary, David and Maria would work together, let's calculate in how many days they would complete the work.

My friend and Rina can do garden work alone in $4$ hours. I can do that work alone in $2$ hours. But if we do that work together, let's calculate how much time will be needed.