Identifying Meaningless Expressions (ME): Mathematical Validation Exercise

To solve this problem, we will evaluate each of the given expressions to determine which one is a meaningless expression due to division by zero:

• First Expression: $0:20$
  In mathematical terms, this expression translates to $\frac{0}{20}$. Division of zero by any non-zero number is defined and equals zero. Therefore, this expression is meaningful.
• Second Expression: $20:0$
  This translates to $\frac{20}{0}$. Division by zero is undefined in mathematics. Therefore, this expression is meaningless.

Hence, the expression $20:0$ is a meaningless expression.

Therefore, the meaningless expression is $20:0$.