Question 1123990
x, volume used for the 90% acid solution


{{{90x+(21-x)97=95*21}}}

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{{{90x+21*97-97x=95*21}}}
{{{-3x=95*21-97*21}}}
{{{x=2*21/3}}}
{{{x=14}}}


The two volume quantities:  14 and 6 liters
Their difference:   8 liters