800:4:20-1*(20*7-50)+0:(705-5)=

Compute step by step (using : as division, left to right):

• 20*7 - 50 = 140 - 50 = 90
• 705 - 5 = 700
• 800 : 4 : 20 = 800/4/20 = 200/20 = 10
• 0 : (705 - 5) = 0/700 = 0

So the expression becomes: 10 - 1*(90) + 0 = 10 - 90 = -80

Final answer: -80