Volum acvariu: 50x50x50=125 000cm³
Arie baza acvariu= 2500 cm²
1dm³ petris=0.6 kg
15kg pietris inseamna 15*1:0.6=25dm³
75 litri=75 dm³
Total cantitate apa+pietris: 25+75=100 dm³=100000 cm³
volum apa+pietris in acvariu (100000)= arie baza (2500) x inaltimea
Atunci :
100 000:2 500=40 cm inaltimea apei