Question 77232
Let x=his sister's age

Then (2x-4)=Ricardo's age

Now we are told that the sum of their ages is 20.  So:

x+(2x-4)=20 get rid of parens
x+2x-4=20  add 4 to both sides
x+2x-4+4=20+4  collect like terms

3x=24   divide both sides by 3

x=8--------------------------------his sister's age

2x-4=2*8-4=16-4=12--------------------Ricardo's age


CK

8+12=20
20=20
and
2*8-4=12
12=12



Hope this helps-----ptaylor