Answer: This is a tough question to answer. There are two ways you can approach it. You can give all the plants exactly the same amount of water. It's hard for me to say how much, however. You might observe the plants, and when you notice one of the plant's soil surface is dry, then water all the plants with the same amount of water. I can't say how much this amount will be--it depends on room temperature and humidity, size of the plants, soil mix, etc.
The problem with this situation is that the plants won't dry out at the same rate, because they are under different light, and possibly heat, conditions. So by watering them all the same, some plants may stay slightly moist while others get soggy and stay that way. Then you don't know if your results are reflecting the soil moisture levels or the light levels.
The other way to approach this is to water the plants individually. That is, check each plant, and water it when the soil surface dries out. This way, you can try to keep all the plants' soil moisture relatively consistent. Then soil moisture won't be a variable.
I would talk this over with your teacher, and decide which approach you want to take.
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