Thank you for the opportunity to help you with your question! the solution is as such:

for the time at which concentration is maxm. d[C(t)]/ dt =0

so t = 150

and at t= 150 , C(t) = 13.5 mg/L

Please let me know if you need any clarification. I'm always happy to answer your questions.awaiting a positive feedback. It was lovely helping you. Please do contact or text me directly for more work assistance. Looking forward to help you again.