b) 99 + (a)   = 1, 000

c) 2 + (a)   = 2, 000

d) 55 + (a)   = 100

f) 800 + (a)   = 1, 600

g) 4 + (a)   = 50

i) 70 - (a)   = 1

k) 3, 000 - (a)   = 1, 100

m) 100 - (a)   = 8

n) 120 - (a)   = 5