Vlook-up dates on the basis of category

I need help on the following:

(Ex A task is started on 30.01.2014 and i need target date of completion on the basis of below:

The above task is consists of 120 Metric Tons

Completion criteria is:

0-50 Metric Tons: 8 days
51-100 Metric Tons: 12 Days
101-150 Metric Tons: 16 Days.... so on

Pl. help.

Regards,
Nagesh.