The number that Albert thought of was 6. This exercise is solved by the crab method, which is applied when the initial quantity is not known, there are several successive operations and the final quantity is known. To solve the inverse operations must be made to those that arise in the problem starting from the end to the beginning.
x + 5 x2 -6 / 2 8
6 -5 / 2 +6 x2 8