Turn over both hourglasses at once. When the four-minute hourglass runs out, turn it over. When the seven-minute hourglass runs out, turn it over. When the four-minute timer runs out this time (eight minutes have elapsed), the seven-minute hourglass has been running for one minute. Now turn over the seven minute timer back over. When the timer runs out, nine minutes have elapsed.
In solving this question it is clear that to pick up the first stone and put it into the basket, the person must walk two yards, one in going for the stone and another in returning with it; that for the second stone he must walk four yards, and so on increasing by two as far as the hundredth, when he must walk two hundred yards, so that the sum total will be the product of 202 multiplied by 50, or 10,100 yards. If any one does not see why we multiply 202 by 50 in getting the answer, we refer him to his arithmetic.