Answer to your second question
It's a simple ratio question. At least I believe that is what it's called. >)
2.6% / 2.4 % = 1.0833333
Now simply multiply 1.08333 with 11000 and you get the answer =11916.66
11000 = 2.6%
X = 2.4%
Here's another one> Weight is 33000 kgs and your gradient is 2.6%. What mass would you achieve with a gradient of 2.9%?