1)
a = (a − b) + b = 120 + 80 = 200;
a + b = 200 + 80 = 280.
2)
a = (a + b) − b = 290 − 90 = 200;
a − b = 200 − 90 = 110.
3)
b = a − (a − b) = 320 − 230 = 90;
a + b = 320 + 90 = 410.

 
1)
d = (c * d) : c = 80 : 20 = 4;
c : d = 20 : 4 = 5.
2)
c = (c : d) * d = 8 * 5 = 40;
c * d = 40 * 5 = 200.
3)
d = c : (c : d) = 80 : 8 = 10;
c * d = 80 * 10 = 800.