1) (1600+t) : 80 = 60
1600 + t = 80 * 60
1600 + t = 4800
t = 4800 - 1600
t = 3200.
2) 2000 : f - 670 =1330
2000 : f = 1330 + 670
2000 : f = 2000
f = 2000 : 2000
f = 0
3) 1300 - (d - 678) = 400
d - 678 = 1300 - 400
d - 678 = 900
d = 900 + 678
d = 1578
4) x + (1200 - 900) = 1500
x + 300 = 1500
x = 1500 - 300
x = 1200.