`1:1`
`F_("Net")=U-W`
`V_(b)d_(b)a_("Nrt")=V_(b)*d_("air")*g-V_(b)*d_(b)*g`
`a_("Net")=(g(d_("air")-d_(b)))/(d_(b))`
`a_("Net")` is independent of volume of the body. Here the two balloons have same pressure, so their densities will also be equal. Therefore, they take same time to reach the given height.