12分米=1.2米
5分米=0.5米
8分米=0.8米
S=1.2×0.5×2+1.2×0.8×2+0.5×0.8×2
=0.6×2+0.96×2+0.4×2
=1.2+1.92+0.8
=3.92﹙平方米﹚
∴3.92×0.25=0.98﹙千克﹚