Marta said 50 + 1/2 m = 120 an equation, while maribel argued that it is an expression. determine who is correct and explain why.

Marta is correct because it has an equal sign, in which 'equa'tions have. An expression in the other hand doesn't have an equal sign.

This here, you don't necessarily include in your answer, it's just to further your understanding:
Expression: 50m+12j
Equation: 6+4x=18
Hope this helps

What is the inequality?