Question 960727
Use the conversion:
{{{ 1 }}} litre = {{{ 1000 }}} cm3
{{{ 15 }}} litres = {{{ 15000 }}} cm3
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{{{ 48 }}} cubes with edge {{{ 5 }}} cm 
have volume:
{{{ 48*5^3 = 6000 }}} cm3
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{{{ 15000 + 6000 = 21000 }}} cm3
Only the depth can be varied, so
{{{ 50*30*x = 21000 }}}
{{{ x = 21000 / 1500 }}}
{{{ x = 14 }}}
The height of the water in the tank is 14 cm