Él no es lo bastante inteligente como para hacer cuentas con la cabeza.

[endycj]

Is “como” necessary here? You can drop it from the sentence, right?

[Bettym]

English Translation

He doesn’t have enough brains to do sums in his head.

I thought como was there to show that it was one example of the way that his intelligence is lacking.

[morbrorper]

como para is kind of a set pattern, used with /lo suficiente/lo bastante/demasiado/tan/lo suficientemente/, etc.