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
