Turing decidable languages On an old worksheet I came across the question

If L1 and L2 are two Turing decidable languages, then show that 1∪2 and 12 are Turing decidable languages (high-level description with stages is enough).

How do I go about answering this without being given a language to work from?
Both $L_1$ and $L_2$ are decidable. Hence, they have algorithms $A_1$ and $A_2$ (respectively) that decide them.
Try to create a new turing machine (algorithm) using the two algorithms $A_1$ and $A_2$.
For example, for the union $L_1\cup L_2$, you can create the following algorithm:

run $A_1$ on the input. If it accepted, then also accept.
else, run $A_2$ on the input, and return what $A_2$ returned.