when air is filled in balloon then it gets inflated. why? In 40 words

With more air molecules in the balloon, the pressure inside the balloon increases. The balloon inflates to increase its surface area so that the force acting by the air molecules inside outward = the force acting by the air molecules outside inward.

However, the pressure inside the balloon is generally higher than outside because there are additional tension force pulling against the balloon’s inflation as it stretches outward.