Textbook Problem

If the expression $ab+cd+\cdot$ in postfix notation is converted to infix notation, what is the result?
b. $\text{Let}\sum \text{=}\left\{1,2,*,/\right\}$and let L be the set of all strings over $\sum$ obtained by writing first a number (1 or 2), then a second number (1 or 2), which can be the same as the first one, and finally an operation (denoted * or /, where * indicates multiplication and /indicates division). Then L is a set of postfix, or reverse Polish, expressions. List all the elements of L between braces, and evaluate the resulting expressions.