70-(81-39) :7+6*7-90:5+85:17

Let's break down the expression step by step according to the order of operations (PEMDAS/BODMAS).

The expression is:

\[ 70 - (81 - 39) : 7 + 6 * 7 - 90 : 5 + 85 : 17 \]

1. Parentheses first: Calculate \( (81 - 39) \). \[ 81 - 39 = 42 \]

2. Now substitute this back into the expression: \[ 70 - 42 : 7 + 6 * 7 - 90 : 5 + 85 : 17 \]

3. Next, perform the division:

   • Calculate \( 42 : 7 \): \[ 42 ÷ 7 = 6 \]
   • Calculate \( 90 : 5 \): \[ 90 ÷ 5 = 18 \]
   • Calculate \( 85 : 17 \): \[ 85 ÷ 17 = 5 \]

4. Substituting these values back into the expression gives: \[ 70 - 6 + 6 * 7 - 18 + 5 \]

5. Next, perform multiplication:

   • Calculate \( 6 * 7 \): \[ 6 * 7 = 42 \]

6. Substituting this value in: \[ 70 - 6 + 42 - 18 + 5 \]

7. Now perform addition and subtraction from left to right:

   • First, \( 70 - 6 = 64 \)
   • Then, \( 64 + 42 = 106 \)
   • Next, \( 106 - 18 = 88 \)
   • Finally, \( 88 + 5 = 93 \)

So, the final result of the expression is:

\[ \boxed{93} \]