Two machines $M1$ and $M2$ are able to execute any of four jobs $P,Q,R$ and $S$. The machines can perform one job on one object at a time. Jobs $P,Q,R$ and $S$ take $30$ minutes,$20$ minutes,$60$ minutes and $15$ minutes each respectively. There are $10$ objects each requiring exactly $1$ job. Job $P$ is to be performed on $2$ objects, Job $Q$ on $3$ objects, Job $R$ on $1$ object and Job $S$ on $4$ objects. What is the minimum time needed to complete all the jobs?
- $2$ hours
- $2.5$ hours
- $3$ hours
- $3.5$ hours