The battery of Amir's phone holds 4000mAh of charge. A notification appears when only 400mAh remains. He always charges it as soon as he can after the notification appears. The phone uses 50mAh per hour of use. Last time his battery notification appeared, which he was able to charge immediately, he only had access to a slow charger that gives 200mAh per 15 minutes, and he charged for 30 minutes. He then used his phone continuously for 6 hours before finding a fast charger that fully charged his battery in one session at a cost of 8p per 100mAh. How much did the fast charge cost?