It depends on what is being referred to by "this."
The description of the activity is that 10 drops are added twice and two readings are taken. If you do *that* 4 times, then a total of 20*4 = 80 drops will have been added.
If the activity referred to by "this" is the pair of acts {add 10 drops, read the volume}, then four repetitions will have added 10*4 = 40 drops.
Probably the expected interpretation is that "this" is {add 10 drops, read}, so 40 drops is likely the expected answer.