We can start by solving for 10-5a+(18-b). By plugging in a and b, we get
10 - 10 + (18 - 11)= 7
28 ÷ (a × 2) = 28/4 = 7
20 + b − (4 × a) = 31 - 8 = 23
49 − 43 + (a ÷ a) = 49 - 43 + 1 = 7
30 − 25 + (b − 9) = 5 + 2 = 7
50 ÷ a − (b + 20) = 25 - 31 = -6
So, 28 ÷ (a × 2), 49 − 43 + (a ÷ a), and 30 − 25 + (b − 9) will all have the same answer as 10−5a+(18−b)
