How to find a variable in a 2 step equation

Okay, say this is your question. " You and three friends go out for dinner. The bill comes to $77. If you pay $20 and your three friends all ordered the exact same thing, what amount will each of your friends have to pay? I used f as the variable to express how much the friends have to pay "
 F=(77-20)/3
subtract what's in the parenthesis first then divide by 3 
F=(77-20)/3
F=57/3
F=19

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