After 10 minutes, he's 40 miles away. After 22 minute, he's 34 miles away. Therefore, in those 12 minutes between, he covered 6 miles. We can make a ratio out of this: 6/12 = 0.5, so he is walking at a rate of 0.5 mi/min (miles per minute). Finally, we can set x/90 equal to 0.5/1 because he is walking at the same rate the entire time. Cross multiply to solve for x. Finally, you get that he had to travel 45 miles in 90 minutes, so his home is 45 miles away from his grandmother's.