Question 315484
Tom is eight years old, so {{{T = 8}}}. 

Sid is 5 years younger than Tom. Younger means that his age is smaller, so we subtract: {{{S = T - 5}}}. 

But we already know that T=8, so we can substitute that into our equation to get {{{S = T - 5 = 8 - 5 = 3}}}, so Sid is 3 years old.