For the series 3 + 7 + 11 + 15 + 19 + ..., what is the fourth partial sum?

1. A partial sum always starts at the first term and runs forward, so the fourth partial sum uses terms one to four. 2. Those terms are 3, 7, 11 and 15. 3. Adding them gives 3 + 7 = 10, then 10 + 11 = 21, then 21 + 15 = 36. 4. So the fourth partial sum is 36. 5. The value 21 is the third partial sum, so stopping one term early is the slip to watch for. _Source: OpenStax Algebra and Trigonometry (CC BY 4.0), Ch 13 "Sequences, Probability and Counting Theory", section 13.4 Series and Their Notations_