What is 18/21 in simplest form?

What's the difference between: x≥0 and nonnegative integer. ?

Suppose that one doll house uses the 1 inch scale, meaning that 1 inch on the doll house corresponds to 1/2 foot for life-sized objects. if the doll house is 108 inches wide, what is the width of the full-sized house that it's modeled after? in inches and feet.

45% of the trees in a park are apple trees. there are 27 apple trees in the park. how many trees are in the park in all?