At the zoo, the ratio of dolphins to penguins is 2:5. If there 30 penguins, how many dolphins are there?

There are 75 dolphins.

Step-by-step explanation:

30 divided by 2 = 15

15 x 5 = 75

Therefore there are 75 dolphins.

I think the answer is 12 dolphins because it 2 dolphins for every 5 penguins

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.