How do you calculate error in refractive index (n)?

N = sin i / sin r

sin i -- incident ray angle
sin r -- refracted ray angle
n -- refractive index

so the answer is 1.49 (3s.f.)
*Refractive Index cannot go below 1

answer and explanation;

composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock while texture refers to the size and arrangement of the minerals or grains that make up a rock. both composition and texture can be used to classify rocks, such as igneous rocks. texture describes how the rock became solid or crystallized from liquid, melted rock. composition of a rock describes the minerals that make up a particular rock.