If the ratio of dolphins to penguins is 2:5, then for every 2 dolphins, there are 5 penguins. If there are 30 penguins, you can use this ratio to find out how many dolphins there are. To find out how many dolphins there are, you can set up a proportion:
(number of dolphins) / (number of penguins) = (2) / (5)
You know that the number of penguins is 30, so you can substitute that in and solve for the number of dolphins:
(number of dolphins) / 30 = 2 / 5
Multiply both sides by 30 to solve for the number of dolphins:
number of dolphins = (2/5) * 30 = 6
So, if there are 30 penguins, there would be 6 dolphins.