Question 203209
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Note the following:


Sister's Age = {{{x}}} -----> <font color=blue>A</font>
Alex's Age = {{{x-8yrs}}} --> <font color=blue>B</font>
Mother's Age = {{{6+2x}}} --> <font color=blue>C</font>


Since Mother's age is given = {{{42 yrs}}}, we can equate to <font color=blue>C</font>.
{{{6+2x=42yrs}}}
{{{2x=42-6=36}}} ---> {{{cross(2)x/cross(2)=cross(36)18/cross(2)}}}
{{{red(x=18yrs)}}}, Sister's Age


Alex's Age= {{{highlight(18)-8=red(10yrs)}}}


To check, 
Mother's Age = {{{6+2(18)=6+36=red(42yrs)}}}  ---> twice Sister's age,{{{2*18=36yrs}}} + 6, {{{36+6=42yrs}}}


Thank you.
Jojo</font>