math bUffs :: cOOl one

Letters 'a', 'b', 'c' &'d' do not appear anywhere in the spellings of 1 to 99
( Letter 'd' comes for the first time in Hundred )

Letters 'a', 'b' & 'c' do not appear anywhere in the spellings of 1 to 999
( Letter 'a' comes for the first time in Thousand )

Letters 'b' & 'c' do not appear anywhere in the spellings of 1 to 999,999,999
( Letter 'b' comes for the first time in Billion )

And
Letter 'c' does not appear anywhere in the spellings of entire English counting