Symbolic puzzle

After the tutorial of Prime 3.1, my answer is attached.  

By the way, I'm not find the answer of Q3 with 6 or 7 unknown marks that Werner_E find the new problem from the same picture!

a+b=13. 5+8  or 4+9

c*d=42.   c, d are 6*7 if the marks are integer numbers from 0 to 9.

e-f=1.     If a+b=5+8 then 4-3 or 3-2 or 2-1 or 1-0.  g is the numbers rest. 

              If a+b=4+9 then 3-2 or 2-1 or 1-0. g is the numbers rest.

Or right side of = sign we have already 1, 2, 3 and 4 in equations. In this case no answer.

Tokoro