Final answer:
Santa can make 6 groups with 3 rocking horse elves in each group, by finding the greatest common divisor of the two types of elves, which is 6.
Step-by-step explanation:
Santa can create several groups with his elves, with each group having the same number of rocking horse elves and the same number of toy car elves. To determine the greatest number of groups he can make, we need to find the greatest common divisor (GCD) for the two numbers of elves. For 18 rocking horse elves and 42 toy car elves, the GCD is 6. Using this, Santa can make 6 groups. There would then be 18 / 6 = 3 rocking horse elves in each group.