n + d = 20 (there are a total of 20 nickels and dimes)
.05n + .1d = 1.4 (total change equals $1.40)
There are a couple of ways to solve this. You can go the elimination method by multiplying the second equation by an amount to get rid of a variable, or we can go substitution by isolating a variable in the first equation. I'll go with elimination in this case:
-10(.05n + .1d = 1.4) this will cause the d-variable to equal 0:
In order to find the volume of a triangular pyramid, you must use the formula (abh)/6. Therefore, I believe the answer is 12 (if [a = a] [b = h] and [c = b]).
