Uni-Trax Publishers allows its sales team to buy and sell shares that pay out $1 only if the sales in the future fall within a certain range. Suppose that, currently, shares for sales between 1,000 and 2,000 textbooks are selling for 10 cents each. Shares for sales between 2,000 and 3,000 textbooks are selling for 30 cents each. Shares for sales between 3,000 and 4,000 textbooks are selling for 40 cents each. Based on this information, what is the probability of Uni-Trax Publishers selling between 2,000 and 4,000 textbooks?