Lin is 7 years younger than adrian,
adrian is 4 years older than half of maya's age,
the sum of the 3 ages is 61,
how old is lin?

Maya is 30 years old, Adrian is 19 years old, Lin is 12 years old.  30 + 19 + 12 = 61.
Start by guessing a number, Then add or subtract what the question says, you will soon have the answer. 

For this item, we let x be Maya's age. By the representation, Adrian age will be,
                                        0.5x +4
And that Lin's age will become,
                                    0.5x + 4 - 7
Summing up all the ages,
                         x + (0.5x + 4) + (0.5x - 3) = 61
The value of x from the equation is 30. Therefore, Lin's age is,
                         0.5(30) - 3 = 12