To solve this expression, you must follow the order of operations: parentheses first, then multiplication, then division, and finally addition and subtraction.

First, solve the parentheses:
66500кг ÷ 19 = 3500кг
3500кг + 15т2ц = 3500кг + 3200кг = 6700кг

Next, multiply 8 by 6700кг:
8 * 6700кг = 53600кг

Finally, subtract 3т20ц from 900т:
900т - 3т20ц = 900т - 320т = 580т

So, the final answer is:
53600кг + 580т = 53600кг + 580000кг = 633600кг