1 is insufficient since you don't have any info on the algorithm of sales.

2 provides the following info: for each $0.25 price increase, the decrease will be 10% of the original sales figure. This can be "translated" into the following:

Say x is the original price of the magazine. If the price becomes x + 0.25*n, then the sales will drop by n*10%. To calculate the drop in number of magazines sold, all you have to do is find n.

Since we know that the price doubled, then we can establish that x + 0.25*n = 2x, meaning that 0.25*n = x. As you can see, unless you know x, then there's no way of finding n.

This is why you also need statement 1: with x = 1.00 = 0.25*n, n will be 4 and the decrease in number of magazines sold will be n*10% = 40%.