#2 Nauka Asana or Boat Pose
This looks like a deceptively easy pose, yet even the most seasoned fitness experts will tell you that this is a killer workout. Keep your legs straight at a 45 degree angle and lift them above the ground.
At the same time place your hands straight, parallel to your legs with your entire core (back and stomach upright). Ideally one should hold this for a minute. Beginners can start with 30 seconds and increase the time period gradually.
While amateurs or advanced fitness enthusiasts can start with 1 minute and continue to increase the time period. Based on the time you have, you can do 3-5 sets of this.
#3 Mountain Climbers
Mountain Climbers are a combination of working your core while also including a bit of cardio and endurance. You are in the same position as a plank except your palms are open and pressed against the ground. You then bend one knee and bring it to your chest and then instantly switch the leg and bring your other knee in.
It is a very dynamic movement because it is almost as though you are running while holding your body up by your arms. Thus its almost like jogging on the spot while the body is horizontal.
Start by doing 50 mountain climbers and do around 3-5 sets of them.
On the whole, combining all three exercises and doing 3-5 sets of them should not take more than 15 minutes in your day. They will work on your different areas of your stomach and strengthen all parts of one's core while focussing on flexibility, strength and endurance. Happy Working Out!