Which set is closed under subtraction?

the set of whole numbers
the set of natural numbers
the set of rational numbers
the set of irrational numbers

the answer is the set of rational numbers

i just took the test, the correct answer is .. 31.5 in.

it's equivalent, so we can say that its equal.

let's call the denominator x and the numerador is 18 less then denominator, x - 18.

multiply

2.x = 5.(x - 18)

2x = 5x - 90

90 = 5x - 2x

90 = 3x

x = 90/3

x = 30

so: