Plan the deliveries

You will model a delivery problem in code and search all the whole-number plans for the cheapest one.

The idea. Depot D1 holds $30$ crates and depot D2 holds $40$. Shop S1 needs $25$ crates and shop S2 needs $35$. Let $x$ crates go from D1 to S1 and $y$ from D1 to S2. Then D2 sends $25 - x$ to S1 and $35 - y$ to S2. The cost per crate in rupees is $3$ from D1 to S1, $6$ from D1 to S2, $5$ from D2 to S1 and $4$ from D2 to S2.

Your tasks.

1. Finish is_valid(x, y). Return True if all four shipments are at least $0$, D1 sends at most $30$ crates and D2 sends at most $40$.
2. Finish total_cost(x, y). Return the total cost in rupees of the four shipments.
3. Finish best_plan(). Try every whole number x from 0 to 25 and y from 0 to 35. Return (lowest cost, x, y) among the valid plans.