Simple mathematical tasks like
13 + 2
3 * 4
can be tackled in 2 ways.
Problem solver can go through all the trouble of calculating the value by mathematical rules.
This method is reliable, although slower, compared to the second representation.
Because of constant training, one is able to solve these equation by retrieving them from memory.
You don't think how much is 2+2. You inherently know it by now.
This approach can also be found with combination of the previous method at solving bigger equations.
Solving 152+66 cannot usually be done from memory. However, after splitting the equation down to digits, the representation returns. Values of 2+6, 5+6 and 1+1(overflow) are retrieved from memory and afterwards combined into end value.
It is debatable if this is a good example of mental representations.
Will have to return to this.