if you drive for 2 hours, at 60 miles per hour, you will have traveled 120 miles. this is a very common type of calculation, involving three quantities: distance, rate and time. if you know just two of these three quantities, you can always calculate the third. thus, if you want to know how much time it is going to take you to get somewhere, and you know that the distance is 120 miles and you will drive at a rate of 60 miles per hour, you can divide the 120 miles by the 60 miles per hour and obtain a time of 2 hours. for this exercise, you will be relating distance, rate and time. the distance we will work with is the depth of happy valley. (remember that even more rock has been removed than the depth of the valley, because the site of the modern valley once was higher than the site of the modern mountain, and the site of the modern mountain has been lowered somewhat as well. and, the rocks had to be deposited, raised and bent before the erosion could occur. so, you're calculating how much time was involved in a small part of the much longer history of central pennsylvania.) we can measure how rapidly rock is being dissolved and washed out of happy valley, and use some fairly simple physical ideas to turn that into the rate at which the valley floor is being lowered. that gives you a distance (the valley depth) and a rate ( the speed at which the valley floor is being lowered), allowing you to calculate the time that has been used in the lowering. to get the correct answer, you will calculate the time from which equation: group of answer choices time