Answer:
Fact: "I bought the rest of it right there at the mall, and I paid a grand total of $7.99 for everything."
Opinion: "The best thing about making a smartphone microscope is that it costs almost nothing."
Fact: "I was able to keep the money I would have spent in my bank account, where it is earning interest every day."
Opinion(?): "Finally, this whole process left me with a terrific tool.”
Step-by-step explanation:
The first one is fact because it states exactly what the person did.
The second one is opinion, it could be mistaken as fact if the making of a "smartphone microscope" actually cost almost nothing but this is unlikely.
The third one is fact because it states what the person did with their money.
The fourth one is most likely opinion because it says they are left with a "terrific" tool, it could just be this person's opinion that the tool is terrific or it could be a figure of speech.(sorry i dont know the right word its probably not a figure of speech but I hope you know I mean)