Question 139512
# of snakes in Jill's yard: j
# of snakes in Bob's yard: b


j + b = 15 (there are a total of 15 snakes -- and maybe you are done right here because this is an algebraic expression for the total number of snakes.  But read on...)


j = 2b (Jill's yard has twice as many as Bob's)


2b + b = 15 (substituting 2b for j)


3b=15