Yes, it is correct, if $t$ represents hours (it is not made clear in the problem what the unit of $t$ is). For instance, if $t$ was in minutes, then you need to substitute $t = 4 \times 60 = 240$. I think it is meant to be hours though.
Be careful what each symbol means. For instance, in the first question, it says "500kg of raw sugar has been refined to 380kg". Does that mean that there is 380kg of raw sugar remaining, and that there is 500 - 380 = 120kg of refined sugar, or the opposite? Or something else? (sorry, I am not familiar with sugar refining) This will affect the meaning of (and answers to) the questions.
In word problems the hardest part is often (at least to me) understanding what the problem is and converting it to math. They often have little traps built-in to confuse people and make them pay attention to wording, it can be frustrating actually.