Question => Is the Christmas sale price of stereo x lower than the Christmas sale price of stereo y.
Let regular price of stereo x = x
Let regular price of stereo y = y
Statement 1 => The sale price of stereo x is 15% less than its regular price; the sale price of stereo y is 20% less than its regular price
x - ( 15% * x ) and y - ( 20% xy) but the value of x and y ( i.e regular price ) is unknown hence statement 1 is NOT SUFFICIENT
Statement 2 => The sale price of stereo x is $150 less than it's regular price; the sale price of stereo y is $200 less than its regular price
x - 150 and y - 200 but the value of x and y ( i.e regular price is unknown ) hence, statement 2 is NOT SUFFICIENT
Combining both statement together =>
$$\frac{\left(Discount\ amount\right)}{original\ amount}=discount\ percentage$$
$$\frac{x-150}{x}=15\%$$ $$\frac{x-150}{x}=\frac{15}{100}=>176$$
$$Also,\ \frac{\ y-200}{y}=20\%;\ =>\ \frac{y-200}{y}=\frac{20}{100}=250$$
Both statements together are SUFFICIENT

Answer is therefore Option C