The tree is as many times taller than the yard stick as the tree's shadow (i.e., the river width) is longer than the yard stick's shadow.
I'm stuck to a proportion problem: A tree which is known to be 75feet high stands on one bank of a river at the waters edge and casts a shadow which just reaches straight across to the other bank, and at the same time a yard stick casts a 4foot shadow. Find the width of the river. Please help, thank you!