bootstrap

Hi all!

I'm having problem in performing bootstrapping with excel, WITHOUT downloading extensions of the program.

My data are the folowinf ones:
I have one insect community made of 3 species, with different abundances:

species A --> 13 insects
species B --> 53 insects
species C --> 20 insects

TOT number of insects in the community: 86

From this community I have to drawn 1000 random samples, and the formula would be:

=INDEX(community;ROWS(community)*RAND()+1;COLUMNS(community)*RAND()+1)

with "community" as my data (13,53,20)

That works well, BUT my problem is that tha sample I have to take don't have to have 86 total insects, but less (50), so I need a random SAMPLE from the above community, for istance a good output would be:

species A --> 35 insects
species B --> 2 insects
species C --> 13 insects

TOT insects: 50

and this would be repeated randomly 1000 times

Do you have any idea how to put the total number of insect constraint?
Hope my headhache will be finally over!!!!!!

Thanks guys!!

Daniele