Question 313306
So for these you have to turn the statements in the word problem into equations. 

A man is three times as old as his son: {{{ M = 3 * S }}} 

his daughter is four years younger than his son: {{{ D = S - 4 }}} 

combined ages of all three is 81 years: {{{ M + D + S = 81 }}} 

Now you can just substitute: {{{ (3*S) + (S-4) +S = 81 }}} 

{{{ 5*S - 4 = 81 }}} 

Add 4 to both sides to get: {{{5*S = 85 }}} 

Divide by 5 on both sides to get: {{{ S = 17 }}} 

Now check: {{{ M = 3 * S = 3 * 17 = 51 }}} and {{{ D = S - 4 = 17 - 4 = 13 }}}, 
so {{{ M + D + S = 51 + 13 + 17 = 81 }}}, so we do indeed get the combined age as 81, which means our son's age of 17 is correct! 

I hope that helps! 

-ntnk