A1:J1 contains 10 maximum test scores A2:J2 contains X's if the test is
supposed to be counted A3:J3, A4:J4, ... contain test scores of pupils (a
blank if the pupil did not take the test)

I now want the cells K3, K4, ... to show the maximum total test score of
the X marked tests the pupil in question took, so for example K9 should be
the sum of the A1:J1 if the cell in A2:J2 has an X and A9:J9 is not blank.