I kept my Dad's and uncle's slide rules, but have never learned how to use them. My dad tried to teach me a couple of times, but it never stuck. I came along at the advent of the calculator. When I was in elementary school, Dad (an algebra teacher for many years) bought an early Sears 4 function calculator. I was allowed to use it to check my math answers - but not to calculate them in the first place. In junior high, I used the printed log tables and by high school my calculator had a "log" button.
And, yes, the answer is 20. Parentheses were not "assumed." They are based on following My Dear Aunt Sal order of operations (M = multiplication, D = Division / A = Addition / S = Subtraction).