A mother's age is $6$ times older than her son

Question

A mother's age is $6$ times older than her son, When her son's age is same as the mother, the sum of their ages will be $85$. What is mother's current age?

Let's start by writing the equations

$$M = 6S$$

$t$ = passed time

$$S + t = M$$

$$S + t +M+t = 85$$

Here we get

$$2M + t = 85$$

which means that

$$12S + t = 85, 12M = 85$$

However, I'm still getting incorrect answers. Can you take a look?

Answer 1

Begin by substituting M in the second equation: $S + t = 6S$. Thus $t = 5S$.
Now substitute t and M in the third equation: $S + 6S + 10S = 85 \rightarrow 17S = 85 \rightarrow S = 5$.
And finally substitute S in the first equation: $M = 30$.

Now check $30 = 6\cdot5$ is right and $30 + 25 + 5 + 25 = 85$ is also right.