A rainstorm increased the amount of water stored in State J reservoirs

Here's another way to look at the numbers in this problem- might help if you feel numbers are a bit inconvenient!

Not getting into the details of the problem, as it's already explained above!

We know that
138 = 0.82T ------- (1)

The other number involved in this problem is 124, which is 14 less than 138

Can you see any relation between 14 and 138? Or rather any relation between 13.8 (which is approximately 14) and 138?
So, you can say 10% of 138 is approx 14. Or, in other words, if you decrease 10% from 138, you get approx 124.

There you go!

Decreasing 10% from both sides of (1) we get: [Note that this is a legit operation - think of it as multiplying both sides by 0.9]
138 - 13.8 = 0.82T - 0.082T (same as 82% - 8%)
124 ~ 0.74T

We are interested in T - 0.74T = 0.26T
If 0.74T = 124, then 0.26T would anyways be greater than 40. (Think of 0.75 ad 0.25 for easy mental calculation!) Only one number in the given options comes close to the answer!

Hope this is clear!

Ambarish