I'm taking a really wild guess, here.....
I think the equations should be of this form......y = C(a/b)^x where C = the percent of water in the wood at x = 0 (in other words, at time 0 ) = 40% or just .40 .....if so......look at both graphs, here
https://www.desmos.com/calculator/qkd7aonn59
For the graph of y = .40(2/3)^x, the time required to dry the wood to 10% water content is about 3.42 hours
Fot the other graph of y = .40(3/4)^x, the time is about 4.82 hours
This makes sense......the first kiln takes the water out of the wood at a faster rate - at least, initially - than the second one .......{we can prove this with Calculus.......}
I believe we have these boundaries:
0 ≤ x ≤ 8 and 0 ≤ y ≤ 4
And we want to know the probability that
x + y ≤ 4 , given the above bounds
See the graph of these regions here......https://www.desmos.com/calculator/dz5d9hoagt
The region bounded by the lines x =0, y = 0, x = 8 and y = 4 has an area of (8)(4) = 32 sq units
And the region bounded by x=0, y=0 and x + y ≤ 4 has an area of (1/2)(4)(4) = (1/2)(16) = 8 units
So the probability that x + y ≤ 4, given the stated bounds = 8/32 = 1/4