I might do this:

```ruby
context "account number must be 10 digits" do
  it "throws error if non-numeric with letters" do
    expect{ Account.new("1234abcd87") }.to raise_error(InvalidAccountNumberError) 
  end

  it "throws error if less than 10 digits" do
    expect{ Account.new("123456448")}.to raise_error(InvalidAccountNumberError)
  end

  it "throws error if greater 10 digits" do
    expect{ Account.new("1234564475558")}.to raise_error(InvalidAccountNumberError)
  end
end
```