Pens cost 95 cent each or 9.95 dollars for a dozens. what is the least possible cost in a dollars of 40 pens

Cost of a pen = 95 cents = $0.95  >statement (1) 
cos of a dozen of pen i.e. 12 pens = $9.95 > statement (2)

Now, Looking at statement (2)
when you buy a dozen of pen, it costs (12 / 9.95) = 1.2069 for each pen
Since, 1.2069 is greater than 0.95, the lowest price for which you can buy a pen is $0.95.

Now,
1 pen costs $0.95
40 pen costs ($0.95 * 40) = $38

So, the least possible cost in a dollars of 40 pens is $38.

Well 0.95 × 40 = 38 so it would be $38 dollars for 40 pens buying separately 
and a dozen is 12, but 12 doesn't fit into 40 properly so we have to buy 48 pens.
12×4=48, 9.95×4=39.8≡$39.80 so that means that buying the pens separately is cheaper.