Rosalie has 30 plants in her garden
Solution:
Given that Elizabeth has
as many plants as Rosalie has in her garden
Also given that Elizabeth has 51 plants
Let "a" be the number of plants in Elizabeth garden
Let "b" be the number of plants in Rosalie garden
So according to given information,
Number of plants in Elizabeth garden =
number of plants in rosalie garden

Thus number of plants in Rosalie garden is 30