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 $x\leq y,x\geq 0,y\geq 0$