= Solution
\b[The printed numerical ratios are about $35$, not about $10$]:
$$
\frac{27.5}{0.78}=35.3,\qquad\frac{27.5}{0.8}=34.4.
$$
The qualitative contrast is nevertheless meaningful. FCC <aluminium> has close-packed <glide planes> and relatively mobile <dislocations>. <Magnesium> also has easy close-packed basal glide, although it lacks enough equivalently easy <slip systems> for arbitrary room-temperature polycrystal deformation.
BCC <iron> has no close-packed plane. Its screw-dislocation cores and lattice-friction barrier give much stronger resistance to glide; thermally activated kink motion makes the critical <stress> particularly temperature- and rate-sensitive. This is the relevant Peierls-stress explanation. A high polycrystal constraint factor cannot explain the supplied contrast, because the data are already <critical resolved shear stresses>. Neither the precise factor35 nor a universal BCC/FCC ratio follows from crystal packing alone: purity, <dislocation> structure, <temperature> and testing conditions also matter.
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