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
You can tell if 2 fractions are equivalent if they are on the same place on this number line. Because 3/8 and 1/2 aren't on the same line, they aren't equal. Contrastingly, because 4/8 and 1/2 ARE on the same line, they are equivalent fractions.