Question 972918
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For pyramid:
a=side of square base; h=height; V=volume
{{{V=(1/3)a^2h}}}
{{{V=(1/3)(10cm)^2(27cm)}}}
{{{V=900cm^3}}}
For tank: 
Volume per mm height=(60cm)(20cm)(0.1cm/mm)=120cm^3/mm
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Fall in level (mm)={{{900cm^3/120cm^3/mm=7.5mm}}}
Fall in level=7.5mm