Every 20 a var goes up by, another goes down by 1

[REEF]

how can the follwing be done in actionscript? when a goes up by 20, b goes up by 1, or when a goes down by 20, b goes down by 1…a little confused here.

a - b
0 - 0

20 - 1

40 - 2

60 - 3

…

280 - 14