That is five hundred and five thousandths.
A thousandth is 0.001 which is 1/1000 as a fraction.
There you've got five hundred and five thousandths which is 505 x 1/1000,
which is ultimately 505/1000.
You can simplify 505/1000 further by dividing both the top and bottom of the fraction by 5.
505/1000 = 101/200