Question 140063
Let the total spinach cans in the case be x

On Monday he ate {{{1/3 *x }}} of the spinach cans.

So amount left after Monday = {{{x - x/3}}} 
                            ={{{ (3x -x)/3}}}

                            =2x/3

On Tuesday he ate 75% of the remaining 
i.e.  {{{(75/100) * (2x/3)}}}

Given that on Tuesday he at 12 cans. Therefore,
{{{(75/100) * (2x/3)}}} ={{{12}}}
{{{(3/4) * (2x/3)}}}= {{{12}}}  simplifying 75/100 as 3/4
{{{x/2}}}= {{{12}}}
{{{x= 12*2}}}
{{{x=24}}}

The amount he ate on Monday = x/3 
                            = 24/3 
                            = 8 cans