Respuesta :
Answer:
Answer:
The correct answer is 15.3 hours
Step-by-step explanation:
See the attached image below to understand why this is correct.

100×20%=20mg is dissolved in one hour
20mg will be dissolved away which mean it will take 4 hours for 80mg to dissolve way
since it takes 20mg 1hour it will take 10mg half and hour or 30min to dissolve away which gives us a total of 4.5 hrs to dissolve 90mg away and leave us with 10mg