Answer: HD = 15
Explaination: If APWVS is similar to HDTFY, we can start out by finding the scale. Given PW and DT are similar, and we have both of their measurments, we can find the scale. 21/35= .6, so this is the scale. Since we have AP is similar to HD and we know that HDTFY is .6 the size of APWVS, we can just multiply 25 by .6. This gets you 15, the length of HD.