You don't say how much the tax is, but if it's included in the "3.5" [$3.50 ??], you have gross sales of 50 x .10 = $5.00
So the net profit is Gross sales - Cost = $[5 - 3.50] = $1.50 ......no rounding necessary!!!!
if you have 250 tootsie rolls
the bag was 3.00 dollars
you sell each one of the tootsie rolls for 10 cents a piece
what is your profit