1300 - (b + 800) = 400
1300 - b - 800 = 400
500 - b = 400
-b = -100
b = 100
1300 - (100 + 800) = 1300 - 900 = 400
600*20 - n = 60
12000 - n = 60
-n = -11940
n = 11940
600*20 - 11940 = 12000 - 11940 = 60
(600 - 500) : y = 20
100 : y = 20
y = 100 : 20
y = 5
(600 - 500) : 5 = 100 : 5 = 20
d : (1300 - 800) = 4
d : 500 = 4
d = 500*4
d = 2000
2000 : (1300 - 800) = 2000 : 500 = 4