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

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

not only in mathematics, scientific proof also are required in the other field of science. also maths proofs are of particular structure, containing steps and logics. so proving is like doing science, it is science. that's why it is important to prove. 

an inaccurate proof leads to errors, or stating results that are not accurate.  which wil mess the probelm up you dont want this