Mary is six years old while her father is y years old. Mary is thirty years younger than her father. Their total age is fifty-seven years. Write a simultaneous equation to represent this information.

Let x be Mary's age and y be her father's age.

From the information given, we can write the following equations:

1) x = 6
2) y = x + 30
3) x + y = 57

Substitute the value of x from equation 1 into equations 2 and 3:

2) y = 6 + 30
y = 36

3) 6 + 36 = 57

Therefore, the simultaneous equations are:
x = 6
y = 36
x + y = 57

Let's define y as the father's age.
According to the given information:
Mary's age = y - 30 (since she is thirty years younger than her father).

Total age of Mary and her father = 57

So, we can write the following simultaneous equation:
y - 30 + y = 57