Permutation and Combination - Arrangements with Objects Separated

Yan is a bit behind and he has 18 tasks to complete on the weekend. Each task takes one hour to complete. Yan knows he has one task in each of Mathematics, Chemistry and Physics. He does not want to work on two of these tasks consecutively. There are no restrictions on the order in which Yan does the other tasks. Find the number of ways Yan can complete his 18 tasks.