A good way to find a quick solution to straighforward math questions can sometimes be to plug-and-chug. Since you know in this case that your answer must be a multiple of 40, you can instantly eliminate choices A and B, since 10^1 = 10 and 10^2 = 100, neither of which are multiples of 40. Also, since they want to know the least value for x, you should continue in this fashion until you find the first multiple of 40. In this case, 10^3 = 1000 and 40 goes into 1000 25 times, so answer is C.