Have is a helping verb, and had is the past participle.
This is why it's okay to use the verb have two times in one sentence.
Hydrogen has 3 Neutrons.
9600=27∗3∗52 So the total number of divisors= 8∗2∗3=48
This question belongs to Present Perfect Continuous Tense.
Simple Sentence Formula: Subject + Have/Has been + Verb + ing + since/for + other words.