Look at these two sentences:
Companies use many methods to expand their business.
Companies use many methods to expand their businesses.
Which sentence is correct? The issue here is about subject-complement agreement. According to Grammar Girl, you can relax. Either option is probably fine because your meaning is usually obvious. It’s likely you mean that any one company has many ways to expand its own business. It’s unlikely that you’re commenting on the many businesses that any one company may run.
Ask students to open their textbook and start reading.
Ask students to open their textbooks and start reading.
Either one of these is OK too. It’s likely you’re implying that every student has one textbook, and it’s the same textbook. It’s unlikely you’re talking about the many textbooks any student may have or that there’s only one textbook and the students are sharing it.
However, if your sentence still seems unclear or crazy sounding, as Grammar Girl puts it, reword your sentence or give other details about how many items you’re talking about. See Grammar Girl’s explanation of subject-complement agreement here.