The general expression for mutation rate is the number of mutant cells over the total number of ...

i really need help with this onw

What have you done so far?

huh?

Please don't do that, i.e. useless posts.

I need this, does somebody know the answer?

from the question we get the information ;

Minimal medium ; with 10;1 dilution of sample we get 18 colonies
minimal medium +leucine ; with the sample dilution of 10;7 we get 9 colonies .
mutation rate ; is the rate of mutation which occurs over period of time . it is given for point muation ,specific class of mutation, insertion theror large scale mutation.
what is the rate of mutation at the locus associated with leucine biosynthesis?

the leucine can be synthesized by the cell in a enzymatic pathway .the locus is situated at the mating site of a cell . leucine biosynthesis requires the enzyme , there is no need for the additional supplement of lucine . as the cells can produce their own leucine . it is normal ongoing processes in the cell

addition of lucine in the medium caused the feed back inhibition ,so the colonies produced is reduced by half ie., 9

the regulation of the lucine biosynthesis is caused by the two way path by two enzymes one is isomerase and another is synthetase. these enzymes are located at the locus point .so there will be muations.