First, let's simplify the expression step by step:

(45-20) = 25
(15-5) = 10

So, the expression becomes:

25 - 10 = 15

Now, let's simplify the next part:

89 - 10 = 79
(20-10) = 10

So, the expression becomes:

79 + 10 = 89

Now, the final expression becomes:

15 - 89 = -74