If we call the number n then 4+12n=4×10=40.
So 12n=40-4=36, and n=36/12=3.
The number is 3.
Another way to solve it is to work backwards. We start with the product of 4 and 10 which is 40.
The product of 12 and the number was added to 4, so we reverse add, which means we subtract 4 from 40 leaving 36.
We're left with the number multiplied by 12, so we reverse multiply, which means we divide 36 by 12 to get 3 which just takes us back to the start, which is the number we started with: 3.