Then, you are right: there are ways in which the boxes can be stacked. Now let's work out how many 's, 's and 's we'll need to make 41.
First, note that there has to be an odd number of 's to make an odd total. So with , we can have and .
Similarly we can have and .
Next, we need to work out how many ways there are of arranging each of these three possible sets of numbers. For this, we need a formula for the number of arrangements with repeated items, which you'll find here.
So with , there are possible arrangements.
With , there are ... ? and with ... ?
To get the probability, add these together and divide by ; cancel the fraction into its lowest terms.
I make it . Do you agree?