Question 281583
First I will label my variables:

J = John
S = Sue
D = David

We know J = S + 3 from John is 3 years older than Sue
We know S = 2D from Sue is twice the age of David

Lastly, we know S = 2D, so why can't the S in J = S + 3 also be 2D

{{{highlight (J = 2D + 3)}}}