Can you help me understand how to review the math definition... what "percentage" means. I need to know what the total mass of the mixture of 42 grams of apples and 96 grams of oranges. ANd what is the percentage of mass the apples are? Thank you.

% anything actually is parts per hundred; i.e., how many somethings you have in 100 total.

If we had 6 toys and 3 were wagons. we would have 50% wagons. That is
% wagons = (# wagons/total toys)*100 = 50%.
Or if there are 75 people in a room and 25 of them are men and 50 are women we have
% men = (#men/total people)*100 = (25/75)*100 = 33.3% and % women is (50/75)*100 = 66.7%.
mass apples = 42
mass oranges = 96
Total mass is 42 + 96 = 138
Then %apples = (42/138)*100 = ?
% oranges = (96/138)*100 = ?
Note the the percentages always adds up to 100% if everything is included.