He will do both in 12 days since the least common multiple of 3 and 4 is 12. This can be found by making a list of both numbers' multiples. You will see that the first common number or multiple is 12.
Multiples of 3: 3, 6, 9, 12, 15 Multiples of 4: 4, 8, 12, 16, 20
6+2/3% <--- Given (6+2/3)/100 <--- Definition of percent (20/3)/100 <--- Putting the top into an improper fraction 20/300 <--- Dividing the fractions 1/15 <--- Simplifying the fraction
