expl3 it's really simple:
\cs_new_eq:NN \calc \fp_eval:n
\calc performs any floating point calculation, which in the above case is rounded to one decimal. Since
\numpoints outputs somewhat like a reference (?? if it doesn't exist and
\exam@numpoints otherwise), I've made an alternative definition
\calcnumpoints which defaults to 0 if
\exam@numpoints is not yet defined. This way you can use it in calculations as expected.