5) (100+39)-10=
=139-10
=129
6)...a)
10*10*10-(100-2*x-6*10)*10=900
(100-2*x-6*10)*10=900+1000
(100-2x-60)*10=1900
100-2x-60=1900:10
2x+160=190
2x=190-160
2x=30
x=30:2
x=15
[(100-y*10):10+5]*10=10*10
[(100-10y):10+5]*10=100
(100-10y):10+5=100:10
(100-10y):10+5=10
(100-10y):10=10-5
(100-10y):10=5
(100-10y)=5*10
100-10y=50
10y=100-50
10y=50
y=50:10
y=5