C = # of carpenter hrs , b = # of blacksmith hrs
c + b = 30...c = 30 - b
20c + 25b = 690
20(30 - b) + 25b = 690
600 - 20b + 25b = 690
-20b + 25b = 690 - 600
5b = 90
b = 90/5
b = 18 <== hrs worked as blacksmith
c + b = 30
c + 18 = 30
c = 30 - 18
c = 12 <== hrs worked as carpenter
