The answer depends on: the height of the item casting the shadow, the location on earth, the time of year, and the inclination of the surface on which the shadow is cast.
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The length of the shadow is proportional to the height of the post. Thus, if l is the length of the unknown shadow, l/17 = 1.2/5 or l = 4.1 feet. This should be rounded to 4 if the value 5 is not considered to be known to at least two significant digits.
Designate the unknown shadow length by s. Shadows cast at the same time and place are proportional to the height of the object casing the shadow. Therefore: 4/6 = s/21, or s = [4(21)]/6 = 14 feet.
Shadow lengths are proportional to the heights of objects casting the shadows. Therefore, calling the shadow length l, the height h, and the proportionality constant k, l = kh. (The intercept is 0 because an object with no height casts no shadow.) Therefore, in this instance k = l/h = 6/3 or 8/4 = 2. then l(6) = 2 X 6 = 12 feet.
If the lamppost is not the light source then lampost's shadow is 112/(64/20) ie 35 inches.
You need more information to solve this problem. The length of a shadow depends on the angle of the sun which depends on the time of day.