A farmer sows wheat on $x$ acres and rice on $y$ acres of his 50 acres. Wheat needs 4 kg of fertiliser an acre and rice 6 kg, and he has 240 kg. Which constraints are right?

Explanation

1. 01

   Write each resource as (amount used per unit) × (number of units), added over the products.

2. 02

   'At most' or 'available' gives ≤, 'at least' or 'must give' gives ≥; a profit is maximised and a cost minimised.