Answer: Well he cant use the quarters because each quarter is 25 cents 25 + 25 = 50
He can use 20 nickels but it will take some time to count them :
5+5+5+5+5+5+5+5+5+5+5+5+5+5+5+5+5+5+5+5=100
But he can also use dimes because every dime is 10 cents and its not to hard to count :
10+10+10+10+10+10+10+10+10+10=100
So I choose dimes