Let x represent the full area if Mary's garden.
Mary plants roses in 1/4 of her garden. This means that the portion of the garden on which she planted roses would be 1/4 × x = x/4
She also plants some tulips in her garden. She has 1/12 of the garden left to plant more flowers. This means that the portion if the garden that she has left to plant more flowers would be 1/12 × x = x/12
The portion of the garden on which she planted tulips would be
x - (x/4 + x/12) = x - (3x + x)/12 = x - 4x/12
x - x/3 = (3x - x)/3 = 2x/3
Therefore, the fraction of Mary's garden that has tulips would be
(2x/3) /x = 2/3