Keith asked 30 people whether they knew how to play whist or dominoes. fifteen of them knoew how to play whist and 20 knew how to play dominoes. five people did not know how to play either game. create a venn diagram to determine how many knew how to play both games.

Ok so, 15 of them know how to play whist (I have no  idea what that is) and 20 of them know how to play dominoes, 5 of them do not. So, if you add up 20+15=35, take off 10 35-10=25 so it must mean that 10 is the answer. (Also if you add 5 to that 25+5=30)
The Answer is 10

Just look outside and then you’ll see where the awnser is

C+19÷-17=17c+(-1.11764706)=17c=.11764706)c=18.1176471