Standard deviation of Mathematics, Physics and Chemistry are 12, 15 and 20 respectively.
Mean of Mathematics, Physics and Chemistry are 42, 32 and 40.9 respectively.
Coefficient of variance of Mathematics,
Coefficient of variance of Physics,
Coefficient of variance of Chemistry,
Since subject with greater C.V. is more variable than others. Thus, the highest variability in marks is in Chemistry and the lowest variability in the marks is in Mathematics.