99.95 + 99.95 percent

irb.rb

irb(main):072:0* counter = 0
=> 0
irb(main):073:0> iterations = 0
=> 0
irb(main):074:0> 1000000000000.times {  iterations += 1 ; counter += 1 if Random.rand(2000) == Random.rand(2000) }
^CIRB::Abort: abort then interrupt!
        from (irb):74:in `call'
        from (irb):74:in `block in irb_binding'
        from (irb):74:in `times'
        from (irb):74
        from /usr/bin/irb:11:in `<main>'

# The above took about 20 minutes. The 11 trillion iterations it managed should be fine

irb(main):075:0> iterations
=> 11490861039
irb(main):076:0> counter
=> 5745685
irb(main):077:0> 100 - (counter.to_f / iterations.to_f)
=> 99.99949997785366