Let your original number be x, and your number after the increase be y.

x + 80%x = y

x(1 + 80%) = y

x = y/(1+80/100)

x = y/1.8

For your given problem,

x = 899/1.8 = 499.44

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If you would rather multiply, you can multiply by 1/1.8 = 5/9 ≈ .55556

x + 80%x = y

x(1 + 80%) = y

x = y/(1+80/100)

x = y/1.8

For your given problem,

x = 899/1.8 = 499.44

_____

If you would rather multiply, you can multiply by 1/1.8 = 5/9 ≈ .55556