correlation =
0.75..........
if corelation lies between 0.7 to 1, it is basically regarded as positively
strong.
correlation between gpa and iq is strong and if iq level increases, gpa level
also increases.
Not only iq level is responsible for gpa level. there are others factors also
like performance, education level etc. so these variables might have influence
on the relationship.
correlation only shows relation -strong or week, it can't use to quantify the
outcome. for example if iq level will be increase by x(say), then how much gpa
level will increase?.. S o for this regression (causation)is needed.

sample size, standard deviation etc are factors that affects the size of
correlation.
to predict gpa, regression is needed because there are others factors that
affects gpa.