- Squirrels and Nuts -- A Black squirrel carries 1 nut at a time and a Grey squirrel carries 2 nuts at a time. If there is a pile of N nuts, how many possible ways can the two squirrels carry these nuts back to their home?
- Trailing zeros -- How many trailing zeros are in 100! ?
- Open lockers -- Which lockers are open after 1000 students open and close them?