Think of this as converting a fraction. 28/40 to x/100. You do this because in the situation, 40 is 100 percent of the price. So 28 is <em>x</em> percent of 100.
You multiply 40 by 2.5 to get to 100, so multiply 28 by the same number to get x. 28 * 2.5 = 70.
So 28 is 70 percent of 40. But the question is asking by what percentage it is <em>discounted. </em>If it's now selling for 70%, it has been discounted 30%. So the answer is 30%
