Question 115205
Let x=ricardo's sister's age
    2x-4=ricardo's age


IF The sum of their ages is 20,

{{{x+(2x-4)=20}}}
{{{(x+2x)-4=20}}}
{{{3x-4=20}}}
{{{3x-4+4=20+4}}}
{{{3x+0=24}}}
{{{3x=24}}}
{{{x=8}}}


Thus, ricardo is 8 yrs old