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a=81-9*6+3*7-38
a=81-54+21-38
a=27+21-38
a=48-38
a=10
b=2*(24-6*3)+6*6-40
b=2*(24-18)+36-40
b=2*6+36-40
b=12+36-40
b=48-40
b=8
c=(15+7*8)-6*4-42
c=(15+56)-24-42
c=71-24-42
c=47-42
c=5
2a-b+3c=2*10-8+3*5=
=20-8+15=
=12+15=
=27