If a balloon is heated, what happens to the volume of the air in the balloon if the pressure is constant?

The volume of a gas's particles is small compared to the overall volume of the gas.

There is an increase in the number of collisions between particles and the walls of the container.

If the volume of a container of gas is reduced, what will happen to the pressure inside the container?

The pressure will increase.

If the balloon is closed, then yes, both volume and pressure will increase when the gas inside is heated ... Now the gas inside the balloon has to create enough pressure to compensate for both the ambient pressure, and the elastic force trying to pull the balloon skin inwards

Basically the volume will get heavier or increase and will then bring you to the sky