Wykonujemy działania:

121 + 4 = 125

125 + 31 = 125 + 30 + 1 = 155 + 1 = 156

156 + 212 = 100 + 200 + 56 + 12 = 300 + 68 = 368

368 + 121 = 300 + 100 + 68 + 21 = 400 + 89 = 489

489 + 5 = 489 + 1 + 4 = 490 + 4 = 494

494 + 203 = 400 + 200 + 94 + 3 = 600 + 97 = 697

697 + 8 = 697 + 3 + 5 = 700 + 5 = 705

705 + 152 = 700 + 100 + 5 + 52 = 800 + 57 = 857

857 + 142 = 800 + 100 + 57 + 42 = 900 + 99 = 999

Uzupełniamy:

$121\stackrel{+4}{\to }125\stackrel{+31}{\to }156\stackrel{+212}{\to }368\stackrel{+121}{\to }489\stackrel{+5}{\to }494\stackrel{+203}{\to }697\stackrel{+8}{\to }705$   

$705\stackrel{+152}{\to }857\stackrel{+142}{\to }999$