Number of zids=x number of zods=y number of zid legs=5x number of zod legs=7y
so 5x+7y=140 try to get it into slope intercept from so you can graph is (y=mx+b) 5x+7y=140 subtract 5x from both sides 7y=140-5x divide both sides by 7 y=20-5/7x y=-5/7x+20
plug in numbers for x and get numbers for y (you can only plug in multiples of 7 for x so that there are a whole number of zids and since you are counting, x and y must never be negative)
lets try 0 for x
y=-5/7(0)+20 y=20 so x=0 and y=20 is an answer (if you can have only one of that species)
lets try 7 for x y=-5/7(7)+20 y=-5+20 y=15
so x=7 and y=15 is an answer
lets try 14 for x
y=-5/7(14)+20 y=-10+20 y=10
so x=14 and y=10 is another answer
lets try 21 for x y=-5/7(21)+20 y=-15+20 y=5
so x=21 and y=5 is another answer
lets try 28 for x y=-5/7(28)-20 y=-20+20 y=0
so x=28 and y=0 is an answer (if there can be only one of a species)
if we go further, then y will be negative so the answers are
(x,y) (0,20) (7,15) (14,10) (21,5) (28,0)
if it is allowed that only one species exists then there are 5 possible answers if both must exist simultaneously, then there are only 3 answers <span />
