Final answer:
After charging the phone for 2.25 hours, it would be at 75% of its full capacity, which is 375 mAh for a 500 mAh battery. When left idle for 3 hours, the phone drains 35 mAh, leaving a remaining charge of 340 mAh.
Step-by-step explanation:
To calculate the remaining charge in the battery of the mobile phone, we need to perform two steps: first, calculate how much charge the phone acquires during 2.25 hours of charging, and second, deduct the charge it loses when kept idle for 3 hours.
Given that a mobile phone gets fully charged in 3 hours, we can assume this to be 100% charge. If 3 hours is 100%, then charging for 2.25 hours (which is ¾ of 3 hours) would result in the phone being charged to 75% capacity. If we assume the full capacity of the battery is 500 mAh (which is not given directly but is necessary to perform calculations), then the phone would have 375 mAh after 2.25 hours of charge (75% of 500 mAh).
The phone then drains 1/56 (approximately 0.018 mAh per minute) of its battery every 9 minutes. Over 3 hours (which is 180 minutes), it would drain 20 times this amount (180/9=20), resulting in a total loss of 0.018 mAh * 20 = 0.36 mAh. Therefore, after being unplugged and kept idle for 3 hours, the phone would lose 35 mAh from its charge, having been 375 mAh initially.
Subtracting the drained charge from the charged capacity, we get 375 mAh - 35 mAh = 340 mAh. Thus, the remaining charge would be 340 mAh, making option a) the correct answer.