Question 1203847

let amount of flour be {{{x}}}

 if he had used the flour to make {{{3}}} cakes, and  {{{3/4}}} of the packet of flour was left
{{{(1/4)x}}}  was  used the flour to make {{{3}}} cakes and
{{{(3/4)x}}}  was left

He went on to bake another {{{5 }}}cakes and had {{{1400g}}} of the flour left. 
if {{{(1/4)x}}}  was  used the flour to make {{{3}}} cakes , then {{{(1/4)x/3=(1/12)x}}} was used for one cake

so, for {{{5 }}} cakes he will use {{{(5/12)x}}}

{{{x-(x/4+5x/12)=1400g}}}

{{{x/3=1400g}}}

{{{x=3*1400g}}}

{{{x=4200g}}}


so,

{{{(1/4)4200g=highlight(1050g)}}}  was  used the flour to make {{{highlight(3)}}} cakes

{{{(5/12)4200g=highlight(1750g)}}}  was  used the flour to make {{{highlight(5)}}} cakes

and
 {{{1400g}}} of the flour left


check:

{{{1050g+1750g+1400g=4200g}}} 
{{{4200g=4200g}}}