Abox contains 10 pairs of shoes (20 shoes in total). if two shoes are selected at random, what is the probability that they are matching shoes?

Once you've selected 1 shoe, there are 19 shoes left
1 shoe will be correct
18 will be wrong

so 1/19 that you get a matching pair

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.