The question states how many ways you can do if at least two students sit together so either you take cases (if 2 students sit together, if 3 sit together, if 4..., if 5... and so on)
OR you do as @devtrivedi did which is much quicker, you do: total - arrangements of the boys all separated (do...
The first statement that “the sum of 50 terms of an arithmetic series is 249” basically means that a1 + a2 + ...... + a50 = 249 (just think a1, a2... a50 as numbers in order in an arithmetic series).
The second statement: “sum of 49 terms of the series is 233” can be written as a1 + a2 + .... +...
1.
a) As Bronsted-Lowry theory states, acids are proton donors and, bases are proton acceptors (here proton can be thought of as H+ since hydrogen itself only has one proton too). So we know sulfuric acid is an acid and is reacting with water, thus the H2SO4 donates a H+ and the water accepts...