Above you declare a `calculator`, but you never use it.

I think you should either:

```ruby
let(:calculator) {Furlong.new}
it "converts 1 furlong to 0.201168 kilometers" do
  calculator.furlong_to_kilometers(1).should be_within(0.0001).of(0.201168)
end
```

OR

```ruby
it "converts 1 furlong to 0.201168 kilometers" do
  subject.furlong_to_kilometers(1).should be_within(0.0001).of(0.201168)
end
```