what type of ride should I choose? I own kross k10 bike. Currently I am doing 7.7 km lap with combination of slow-fast-slow (40%-20%-40%) of total path.
Am I doing it correct?
When you say "to burn tummy fat", presumable you're talking about weight loss and general toning?
What worked for me was a short, 10-minute / 2-point-something km commute, twice a day, five days a week, over two or three years. Because the ride was so short I was able to give pretty much 100% for the whole ride, with natural stops at red lights.
I went from maybe 110kg down to about 80kg in this time. It was kinda counter-intuitive - in that if you just think about calories I burned during the ride, the shortness of the ride should not have made too much difference. But I lost weight. I'm now lighter still, but that loss has happened over a far longer period and is more regulated.
The way I knew I was losing weight was by clothes (including weird stuff like shoes, hats and my wedding ring) becoming looser. I did not weigh myself at first - I do these days and with hindsight I should have got some scales sooner. Without measurement, it is certainly possible to convince yourself that nothing you are doing is making any difference.
So, for my money, you need some degree of intensity there. I'm not sure that cycling anywhere without much effort (which I suspect is what you mean when you say "slow") will do much to help weight loss. But by the same token you don't necessarily need to be in the saddle for hours. Also it is a gradual effect and takes time.
On top of all that, if you want to specifically develop tummy muscles, I'd recommend some other form of training as well. Cycling is great for aerobic exercise and for developing your legs, but leaves something to be desired as regards upper body muscles. Some professional cyclists, for example, will work with weights a couple of times a week, especially in the off-season, to keep themselves at peak fitness.
You can't really burn "tummy fat." Your body has an order that it will store fat in and it will lose fat in the reverse order. You can lose bodyfat, but it goes away in the reverse order that you put it on. You can make it appear that you've proportionally lost more weight in your tummy by building up the muscles there.
I'll second Eric's answer, but with a little more detail.
There is no way to exercise that will produce "spot removal" of fat. The only way to do that is via liposuction. Each person's body will deposit fat differently. For instance, my body starts with deposits on my lower back, then on my belly, and then my upper arms and face. Other people start on their belly or upper arms. If you reduce or increase your body fat percentage slowly over time, you can easily observe the pattern your body uses. You can also see it in videos of the morbidly obese, who have massive variety in how their fat gets stored.
If you want to lose tummy fat, the best bike ride is the one that burns the most calories, since (provided you don't eat to make up for the lost energy) this will cause the burning of fat. Energy expenditure is just time * intensity, and there are lots of good calculators for this floating around online. For the untrained cyclist, I think they're pretty accurate (trained cyclists can be more efficient).
Short, hard rides are probably a good choice if you want to lose weight, because they take less time per calorie spent. 60 minutes as fast as you can maintain it is a good choice, because it's a nice length of time where you shouldn't need a snack, even at maximum effort. At first you may not be able to go fast if you lack cardiovascular strength, but even at a walking-level effort you should be able to cover about 12-15km in that time, about twice the distance you're doing now. For a typical person, this would burn maybe 500 kcals. Then just work on going faster (and therefore further) each time you go. Doing that level of exercise daily should result in the loss of a 1lb of fat/week, provided you don't eat more to make up for it.