Question 151619
In order to solve this problem, one must assume that there are no 1-legged people or 3-legged elephants in the group, i.e. all of the people have 2 legs and therefore 2 knees, and all of the elephants have 4 legs and therefore 4 knees.  You also have to assume that there are no elephants which, for whatever reason, have a different number of trunks than 1.  Given all of that:


x is the number of people
y is the number of elephants


Then:
2x is the number of knees attributible to people and
4y is the number of knees attributible to elephants.


Since there are 100 knees, {{{2x + 4y = 100}}}


By similar analysis, {{{3x+y=100}}} is the equation that represents the distribution of trunks of both kinds (elephant proboscises and containers for personal belongings).


Your task is then to solve this system of two linear equations in two variables.  I recommend the elimination method for this problem.


Hint:  Multiply the 2nd equation by -4.