00:03
Hi there.
00:04
In this question, we have a mixture of pennies of just two types.
00:08
So if we think about our entire sample, we have 100%.
00:13
And we are told that 46 .7 % are pre -1982.
00:26
Well, since we only have pre -1982 and post -1982, subtracting 46 .7 % from 100%, which represents all of our pennies, tells us that we have 53 .3 .3 .3.
00:40
Percent post -1982.
00:48
And the pre -1982 pennies have a mass of 3 .1 grams each, whereas the post -1982 have a mass of 2 .5 grams each.
01:04
All right, and we want to know the average mass of a penny.
01:08
So what we're doing here is calculating a weighted average.
01:11
And this is the way that atomic masses are calculated on the periodic table.
01:16
The atomic mass on the periodic table is an average.
01:20
Atomic mass.
01:22
And like i say, it is a weighted average.
01:24
You probably have the most experience with weighted averages when calculating your grade...
