Billy has $3.00 and buys 4 pieces of bubble gum at $0.30 each.
First, calculate the total cost of the 4 pieces of bubble gum:
\[ \text{Total Cost} = 4 \times 0.30 = 1.20 \]
Now, subtract the total cost from the amount of money Billy initially had:
\[ \text{Money Left} = 3.00 - 1.20 = 1.80 \]
So, Billy has $1.80 left.