I have 18 bills that equal $100.00 (5-$10 bills, 9-$5 bills, 3-$1 bills, 1-$2 bill). How can this be expressed in an algebric equation?
Usually algebraic equations contain variables that represent some kind of unknown quantity.
In your case there is no unknown, you know all there is to know. There is no problem stated.
To express this situation in an algebraic equation, we can assign variables to the different denominations of the bills. Let's use the variables T for $10 bills, F for $5 bills, O for $1 bills, and W for the $2 bill.
Given the number of each bill:
T = 5 (number of $10 bills)
F = 9 (number of $5 bills)
O = 3 (number of $1 bills)
W = 1 (number of $2 bills)
We can express the total value of the bills in terms of these variables:
10T + 5F + 1O + 2W = 100
So the algebraic equation that represents this situation is:
10T + 5F + O + 2W = 100