If you insist on converting 3/5 to a whole number, the best we can do is round the decimal number up or down to the nearest whole number. 3/5 rounded up to the nearest whole number is 1 and 3/5 rounded down to the nearest whole number is 0.
You will know it is an inch ruler because it will have 12 lines that denote inches on ... In the first inch, these marks will mark 1/4, 1/2, 3/4, and 1 ...