1. The problem asks to find which expressions have a sum or difference equal to $\frac{3}{4}$.\n\n2. We will evaluate each expression by finding a common denominator and simplifying.\n\n3. Expression 1: $\frac{1}{2} + \frac{2}{4}$\nCommon denominator is 4, so $\frac{1}{2} = \frac{2}{4}$.\n$$\frac{2}{4} + \frac{2}{4} = \frac{4}{4} = 1$$\nThis is not $\frac{3}{4}$.\n\n4. Expression 2: $\frac{11}{12} - \frac{1}{6}$\nCommon denominator is 12, so $\frac{1}{6} = \frac{2}{12}$.\n$$\frac{11}{12} - \frac{2}{12} = \frac{9}{12} = \frac{3}{4}$$\nThis equals $\frac{3}{4}$.\n\n5. Expression 3: $\frac{3}{5} + \frac{3}{20}$\nCommon denominator is 20, so $\frac{3}{5} = \frac{12}{20}$.\n$$\frac{12}{20} + \frac{3}{20} = \frac{15}{20} = \frac{3}{4}$$\nThis equals $\frac{3}{4}$.\n\n6. Expression 4: $\frac{7}{8} - \frac{1}{2}$\nCommon denominator is 8, so $\frac{1}{2} = \frac{4}{8}$.\n$$\frac{7}{8} - \frac{4}{8} = \frac{3}{8}$$\nThis is not $\frac{3}{4}$.\n\n7. Expression 5: $\frac{2}{3} + \frac{1}{12}$\nCommon denominator is 12, so $\frac{2}{3} = \frac{8}{12}$.\n$$\frac{8}{12} + \frac{1}{12} = \frac{9}{12} = \frac{3}{4}$$\nThis equals $\frac{3}{4}$.\n\nFinal answer: Expressions 2, 3, and 5 have a sum or difference of $\frac{3}{4}$.