Assuming that profit of a shopkeeper in a particular commodity is a linear expression of transportation charge (t) and the quantity of commodity (q). He earns a profit of Rs. 10,000 by selling 20 units at the transport charge of Rs. 400. He also earns a profit of Rs. 12,000 by selling 25 units at the transport charge of Rs. 600. What is the linear expression in t and q?