Step 1:
The data in the given problem can be summarized as follows:
For the type of large vans:
Packages =200
Cost = Rs.400
For the type of small vans:
Packages =80
Cost = Rs.200
For the minimum requirement :
Packages =1200
Cost = Rs.300
Step 2:
Let $x$ be no of large vans and $Y$ be the no of small vans,used for carrying the packages.The objective is to minimize the cost.
The mathematic formulation of the above problem is as follows :
Minimize $z=400x+200y$ subject to
$200x+80y\geq 1200$
$\Rightarrow 5x+2y\geq 30$
$400x+200y\geq 3000$
$\Rightarrow 2x+y\geq 15$
Also $x\leq y,x\geq 0,y\geq 0$