1. To find the least common denominator, you need to find the least common multiple of the two denominators which are 6 and 15. Prime factorization of these numbers gives:
6 = 2 x 3
15 = 5 x 3
A number that would evenly divide both 6 and 15 must contain 2 x 3 x 5 which is 30.
Thus 30 is the least common denominator.
2. Now we need to somehow change both fractions to make them have a denominator of 30. To do this we can multiply by fractions with same numerator and denominator since that would be like multiplying by 1.
Well if he made 10 in 4 hours, that means he made 5 in 2 hours. after 2 hours he had 30 cards, so that means he started with 25 cards. Mr. Cringle is making 2 and 1/2 cards per hour.
