can you help me understand how to answer this

(c) A factory, mass-produces a cheap device. It is known that about 1 in 10 are 'poor'. These , devices are sequentially counted and tested until the next poor one is found. This num-' beris recorded. What is the probability that this number is in the standard interval, adopting an appropriate probability model. ' Hint: The geometric randorh variable, N ~ QeUJ), 0