We know that college enrollments drop by 10% when textbook prices double. This means that as the price of textbooks increases, fewer people attend college. This is a standard definition of complementary goods, so the cross price elasticity must be negative. More than that though, we know a doubling of price (a 100% increase) leads to a reduction in enrollments of 10%. From the general formula for cross price elasticity we have:

17. If college enrollments drop by 10% when textbook prices double; textbooks and enrollments are __________ goods and their cross price elasticity is __________.

1. complementary; -0.5 Complementary is correct, but the cross price elasticity is wrong. College enrollments would have had to drop by 50% for this to be correct.
2. substitute; 5 Substitutes is wrong, as is the cross price elasticity. College enrollments would have had to increase by 500% for this to be correct.
3. complementary; -0.1 Correct, as shown above.
4. substitute; 0.5 Substitutes is wrong, as is the cross price elasticity. College enrollments would have had to increase by 50% for this to be correct.
5. complementary; -5 Complementary is correct, but the cross price elasticity is wrong. College enrollments would have had to drop by 500% for this to be correct.