Question 789602
{{{b}}}= age of the brother, in years, at the start of the 3 years
{{{b+10}}}= age of the sister, in years, at the start of the 3 years
{{{b+3}}}= age of the brother, in years, after 3 years
{{{b+10+3=b+13}}}= age of the sister, in years, after 3 years
 
If after 3 years the sister is twice as old as the brother,
{{{b+13=2(b+3)}}}
Then, solving step by step:
{{{b+13=2(b+3)}}}-->{{{b+13=2b+2*3)}}}-->{{{b+13=2b+6)}}}-->{{{b+13-6=2b)}}}-->{{{b+7=2b)}}}-->{{{7=2b-b)}}}--> {{{highlight(b=7)}}}
That means:
{{{b+10=7+10=highlight(17)}}}= age of the sister, in years, at the start of the 3 years
{{{b+3=7+3=10}}}= age of the brother, in years, after 3 years
{{{b+13=7+13=20}}}= age of the sister, in years, after 3 years
At the start the brother is 7 years old, and the sister is 17.
After 3 years, the brother is 10, and the sister is 20.