Basmati rice costs $$ $(a−b)$$ per kilogram in standard 7 kilogram packages, where $$a\gt b\gt 0$$, and costs $$ $a$$ per kilogram in nonstandard packages. In terms of $$a$$ and $$b$$, what is the lowest possible price of exactly 24 kilograms of Basmati rice, in dollars?

Incorrect.
[[snippet]]

Incorrect.
[[snippet]]

Incorrect.
[[snippet]]

Incorrect.
[[snippet]]

That's right.
The variables in the question and in the answer choices call for __Plugging In__. Use good numbers such as $$a=5$$ and $$b=3$$.
Now the question reads as follows:
>{color:purple}Basmati rice costs __$2__ per kilogram in standard 7 kilogram packages and __$5__ per kilogram in nonstandard packages. What is the lowest possible price for 24 kilograms of Basmati rice in dollars?{/color}
How does it feel now? Probably much better….
Exactly 24 kilograms are made of 3 standard packages of 7 kilograms, plus 3 kilograms, hence
>$$ $2\times 21 + $5\times 3 = $57$$
Your goal is __$57__. __Plug In__ $$a=5$$ and $$b=3$$ in the answer choices, and eliminate any answer choice that clearly is not 57. The correct answer is C.

$$24a-3b$$

$$21a-3b$$

$$24a-21b$$

$$21a-21b$$

$$21a-24b$$