Answer:
An egg and cheese biscuit was $3.65.
Explanation:
Liam's breakfast was:

Where:
P: total price
E: egg
C: cheese biscuit
M: carton of milk

(1)
Now, Finn's breakfast was:

(2)
By subtracting equation (2) from (1) we have:


Hence, the price of a carton of milk is $2, so by entering this value into equation (1) we can find the price of an egg and cheese biscuit:


Therefore, an egg and cheese biscuit was $3.65.
I hope it helps you!