Question 301653
There are 2 similar right triangles:
One is formed by her and her shadow
The other is formed by the streetlight
and (distance to her + her shadow)
I can write a ratio of the sides 
of these similar triangles:
Let {{{x}}} = the height of the streetlight
{{{5.5/7 = x/(21 + 7)}}}
{{{5.5/7 = x/28}}}
multiply both sides by {{{28}}}
{{{4*5.5 = x}}}
{{{x = 22}}}
The streetlight is 22 ft high