There is insufficient information to tell how many are studying just two of the three subjects, but it is possible to put limits on the possible number

Minimum number studying just two subjects ZERO

Maths alone 25

Physics alone 20

Chemistry alone 10

All three 15

TOTAL 70

Maximum studying just two subjects FORTY FIVE

(this is ONE solution that has 45 studying just 2 subjects, there are others)

Maths and Chemistry 25

Physics and Chemistry 20

Chemistry alone 10

All three 15

Other combinations of subjects give rise to different numbers studying just 2 subjects