Two independent uncertainties of 0.3 and 0.4 units combine in quadrature. Compute the combined uncertainty, to one decimal place.

0.5 (answers within ±0.02 count)

$\delta = \sqrt{0.09 + 0.16} = \sqrt{0.25} = 0.5$. Note the total is well below the arithmetic sum of 0.7, and that the larger term dominates, so effort is best spent reducing it.