What is the proportion in the first question?

If two triangles are similar, the correspondant sides are in proportion. So, the proportion is

12 feet because you take 10 feet and multiply by 12 to make it inches then divide that by 15 to find the scale factor and it is 8 so you take 18 and multiple that by 8. You get 144. You then divide that by 12 to convert it back to feet and you get 12 feet.