From a pile of 100 pennies(p), 100 nickels(n), and 100 dimes(d), select 21 coins which have a total value of exactly $1.00. in your selection you must also use at least one coin of each type. how many coins of each of the three types(p, n, d) should be selected?

So, you need 10 penny, 4 nickles and 7 dimes
Lets, check it.
According to the question,
total coins = 21
7 + 4 + 10 = 21
21 = 21true
Now,
their value should equal $1.00 = 100 cents
Penny = 1 cent
nickle = 5 cent
dime = 10 cent,
So,
(10 *1) + (4 * 5) + (7 * 10) = 100
10 + 20 + 70 = 100
100 = 100true

 
So, you need 10 penny, 4 nickle, and 7 dimes

Im pretty sure it’s the first one