Question 918962
Let {{{ p }}} = price per unit before going up
{{{ p + .2p = 1.2p }}} = price per unit  after going up
I can say they consumed any amount before
the price rose. I will say they consumed
{{{ 100 }}} units ( any units )
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Let {{{ k }}} = the fraction to reduce consumption by
{{{ 100p = ( 100 - 100k )*1.2p }}}
{{{ 100p = 100*( 1 - k )*1.2p }}}
Divide both sides by {{{ 100p }}}
{{{ 1 = 1.2*( 1 - k ) }}}
{{{ 1 = 1.2 - 1.2k }}}
{{{ 1.2k = .2 }}}
{{{ k = 1/6 }}}
She needs to reduce consumption by 16.67%
check:
Say {{{ p = 7 }}} 
{{{ p + .2p = 1.2*7 }}}
{{{ 1.2*7 = 8.4 }}}
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Say they consume {{{ 11 }}} units
{{{ 11*7 = 77 }}} is the cost of milk before increase
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Then they consume {{{ 11 - (1/6)*11 = 55/6 }}} units
after increase
{{{ 8.4*(55/6) = 77 }}}
Their cost still is the same after increase in price -OK