To write 6.875 in words, you would say "six and eight hundred seventy-five thousandths." This breaks down as follows: 6 is "six," 0.8 is "eight tenths," and 0.075 is "seventy-five thousandths." So when combined, it is "six and eight hundred seventy-five thousandths."
Well, darling, to write 6.875 in words, you would say "six point eight seven five." It's as simple as that, sugar. Now go impress someone with your newfound ability to spell out decimal numbers.