Writing inequalities
What we know?
Budget: $40
Fruit: $4/pound
Granola bars: $1 each
At least 20 granolas
Let's call
f: number of pounds of fruit
g: number of granola bars
Therefore
The total cost of fruit is given by: $4 · f
The total cost of granola is given by: $1 · g
The total cost of everything is given by: $4 · f + $1 · g
Since budget: $40 then
$4 · f + $1 · g = $40
4f + g = 40
g > 20 means we have at least 20 granolas
4f + g > 20 means that we have at least 20 granolas, that cost $20, since 4f + g = 40 and 40 > 20