In response to arunpv:

No, you are not doing the maths correctly - you are looking at things in too simplistic a way. Throwing more devs at a problem and/or making them work longer hours WILL result in longer delays... this is not just theory; it's backed up by some pretty substantial evidence that has been collected over the years, is readily available with a little searching, and has been written about ad nauseum in more project management books than you can shake a stick at.

There are many reasons...

You realistically can only get 50-60 hours per week out of someone. Every hour beyond that point is less and less efficient until you get to the point that he's working full out but has no traction and isn't making headway.

Also consider that every dev you add to a project increases the lines of communication that must be maintained by n-1, where n is the number of assigned developers. Each line of communication is a bottleneck and a productivity loss.

... and it's not simply software development that realises these truisms:

We're not talking about picking berries in a field where the speed at which work can be accomplished is directly proportional to the number of people assigned. Software development on complex systems has many artificial constraints that limits the usefulness of adding more hands to the job... components that don't lend themselves to being broken up across devs, integration points for components that must inter-operate, etc.

The best analogy I can think of right now is a kitchen... throwing more cooks at the preparation of a meal doesn't make it happen any faster because there are only so many ovens available, only so many pots to use. To get the meal out on time requires a concerted effort to ensure that everything comes together at the right time; that the potatoes weren't finished 15 minutes before the roast and are sitting on the counter going cold. Does the roast cook any faster just because it was put in the oven by 4 cooks instead of 1? Of course not... the time required is constrained by artificial bounds... the temperature of the oven and the physical properties of the meat. I know it's a bit of a confused analogy, but hopefully you see my point?