While in Wonderland, Alice eats a cookie that makes her grow taller at a rate of 0.5 ft/min. She is standing 40 feet away from a light which is 20 feet tall. How fast is the length of her shadow changing when she is 15 feet tall?

I know that this is a similar triangles problem and you have to cross-multiply the ratios, but that's all that I know. Can someone please help me? Thank you so much!