Virus on rooted phone / how to delete all files and reinstall android

Hi there! Thanks in advance for looking at this.

Im a regular phone user with no experience of rooting phones but fairly tech literate and could probably learn if necessary.

I purchased an samsung s3 mini for my girlfriend from CEX (second hand phones), only to find out later that it seems to be rooted with some malware on it. After a factory reset, still loads of apps remain and some pop up problems persist. I managed to install CM security which removed 4 threats but theres still a load...

Virus on rooted phone / how to delete all files and reinstall android

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