To check, if you use the standard Windows 'Calculator' program to work out the sum you will get two different calculated values depending on how the Calculator 'View' is set as either 'Standard' or 'Scientific'.
Last time a "debate" like this happened (here, a debate between the people who say that it's 20 and the people who are wrong), I'm fairly sure I managed to construct a formal "proof" for why multiplication precedes addition. I'll see if I can dig it up and find it. In the meantime, the answer is 20.