Is this an effective way to lose 20 pounds in the month of August?

Hi guys, I'm 14, currently 165 pounds. During the last two months, I lost about 20 pounds, but I really don't know exactly how I did. Now I want to be AT LEAST 150 pounds before September starts. What I do is 50 push ups (increases by week), 40 sit-ups (increases by week), and some leg exercises but they aren't important because I'm trying to focus on my stomach area. I do these exercises around 12:00 p.m. and again at around 8-9:30 p.m. Everyday I'm currently running for 30 minutes at 6 m.p.h. (on a treadmill). I increase the time by 5 minutes each week. Is this a good way to get the pounds I want burned off? Thank you for anyone who can help me with this problem.