Lucas is offered either 15% or 21$ off his total shopping bill. how much would have to be spent to make 15% option the best one?

Assume that the amount of money that Lucas would have to spend is "x".

Now, Lucas is offered:
either 15% of his total shopping which can be written as 0.15x
or 21$

We want the 15% to be a better option, therefore:
0.15x > 21

Solve the inequality for x:
0.15x > 21
x > 21/0.15
x > 140

Therefore, for the 15% back to be a better option, Lucas has to spend more than $140