Assume E represents the cost of an egg and T represents the cost of one piece of toast. 
We can construct two equations :
2E + T = $3.00 - - - - - (a) 
E + T = $1.80     - - - - - (b) 
Subtract equation b from a:
E = $1.20 (The price of one egg) 
To find out the price of one piece of toast, replace the price of one egg in equation b:
T = $1.80 - $1.20 = $0.60 (The price of one piece of toast) 
Hope that helps you