Problem: Let n be the number of ways that 10 dollars can be changed into dimes and quarters, with at least one of each coin being used. Then n equals:
Answer Choices:
A. 40
B. 38
C. 21
D. 20
E. 19
Solution:
25q+10 d=1000,q=40−52 d∴40>52 d, d<100. But d=5k
∴5k<100∴k≤19∴n=19