Algebraic approach:
Statement 1Let \(x\) be the discount Bill received and let \(y\) be the previous period's discount. So we get this equation:
\(450(1-x) = 450(1-y) - 112.50\)
Which reduces to:
\(x-y = 0.25\)
This means that in the previous period, the discount was 25% less. Thus, Bill either bought his suit on July 8 or July 15. July 1 is not possible because there was no discount the day before. And July 22 is not possible because the discount was 15% less the day before.
Not sufficient.
Note that if the equation reduced to \(x-y = 0.15\), Statement 1 alone would have been sufficient.
Statement 2\(202.50 = 450(1-x)\) where \(x\) is the discount Bill received.
\(x = 0.55\)
So Bill could have bought his suit on any day July 15 through July 21.
Not sufficient.
Combining both statementsIn Statement 1, we narrowed down the answer to July 8 or July 15.
In Statement 2, we narrowed down the answer to July 15 through July 21.
The only date common do both is July 15.
Sufficient.