Manipulating percentages.

Hi,

I appreciate any help you can give to this problem i have.

the end value must be no more than 100%

a = 70%

b = 10%

c = 10%

if i add all the values together i get 90%, which leaves 10%. this value CANNOT exceed a value that is being changed but the end value must add upto 100%

i want to change c to 20% so it all adds up to 100% but heres the problem...

i cant change c to 20% because the the value i got from adding all of the values together and taking away 100% is 10% and 20% is more than 10%

your help is appreciated