Do not think that this question pertaining much more to quant rather than to verbal. However, we can apply some gmat-tricks from quant section (pick numbers)
Say that in 1982 there were 90 low-paid , 5 mid-paid guys and 5 high-paid guys.
We want to keep proportion of low-paid guys constant.
In order to do that, just multiply by 2 each number.
In 1985 - we will have 180 low-paid , 10 mid-paid, 10 high-paid
(Please note that proportion of low-paid remained the same = 90%)
now we need just to slightly play with numbers (in order to comply with presented argument)
Let's have 180 low-paid 0 mid paid 20 high-paid guys in 1985
From 1982 to 1985, the number of low-paid guys had increased by 90 (however proportion remained 90%), the number of high-paid had increased just by 10 (however proportion had increased from 5% to 10%)
As you can see - now we comply with argument!
A just describes our example!