12-40:k=4
40:k=8
k=5 (30-n):2=9
30-n=18
n=12 7*b-15=6
7b=21
b=3 20:(x:9)=4
x:9=5
x=45 (9*t+37):8=8
9t+37=64
9t=27
t=3
