To solve this expression, we need to follow the order of operations (PEMDAS).

First, calculate what's inside the brackets:
300 + 200 = 500

Next, calculate the expression within the brackets:
500 ÷ 500 = 1

Now, using this result, we can proceed with the rest of the operations step by step:
1 × 60 = 60
60 - 100 = -40
-40 × 10 = -400
-400 ÷ 5 = -80
-80 × 50 = -4000
-4000 ÷ 5 = -800
-800 + 100 = -700
-700 - 500 = -1200

Therefore, the final answer is -1200.