How do I always round a number down in Ruby?

For example, if I want 987 to equal "900".

n = 987
m = 2

n.floor(-m)
  #=> 900

See Integer#floor: "When the precision is negative, the returned value is an integer with at least ndigits.abs trailing zeros."

or

(n / 10**m) * 10**m
  #=> 900