If Jenny planted 100 rose flowers in total that would mean she planted 25 jasmine flowers.
Think of it in this way: every 4 rose flowers, she plants 1 jasmine flower. So if you draw 100 little dots (those represent the rose flowers) then every four dots draw a circle around them. In each group there should be 4 dots in each. And if you count each group you will see there are 25 groups.
So for the 100 rose flowers she planted she also planted 25 jasmine flowers.