Physics conceptual question

Do they start out at the same volume? If so, your assumption is right. If they're different sizes, then it depends on the size of the rigid balloon.

Thanks!

That is all the information I am given. I am assuming that they start out with equal volumes.

Actually, the key point is not that its volume increases, but that its density decreases as the air pressure around it declines. Lower density will mean the balloon is displaced upwards by the denser air around it. By comparison, the rigid balloon will reach some stable height where the average density of the balloon/helium combo is the same as the air around it, and it will stay there (ignoring random pressure or temperature differences through the air and over time, of course).

Old Prof wrote:

Actually, the key point is not that its volume increases, but that its density decreases as the air pressure around it declines. Lower density will mean the balloon is displaced upwards by the denser air around it.

hates physics wrote:

thus it's volume increases and it increases the buoyant force of the balloon.

The two points are one and the same. The OP said the weight was a given, so the density and volume are inversely proportional. You can look at it in terms of a decreasing density, or see it as a constant weight displacing a larger volume (and thus a larger mass) of air.