Explanation
This answer is based on the formula of schedule performance index (SPI), which is the ratio of earned value (EV) to planned value (PV). Earned value (EV) is the measure of work performed expressed in terms of the budget authorized for that work. Planned value (PV) is the authorized budget assigned to the scheduled work.
In an agile project, story points are often used as a unit of measure for expressing the size of a user story,feature, or other piece of work. Therefore, in this question, the earned value (EV) is 30 story points, which is the actual work completed in the iteration, and the planned value (PV) is 40 story points, which is the planned work for the iteration. By applying the formula of SPI = EV / PV, we get SPI = 30 / 40 = 0.75. This means that the project is behind schedule, as the SPI is less than 1. References: (Project Management Professional (PMP) Reference Materials source and documents) A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Seventh Edition, Chapter
7: Planning and Managing the Budget, Section 7.3: Managing the Budget, page 172.
Agile Practice Guide, Chapter 5: Implementing Agile: Monitoring and Controlling, Section 5.2:
Monitoring and Controlling in an Agile Environment, page 58.