Permutations and Combinations Part 2

‌a∈{2,4,6,8,10,....,100}
‌b∈{1,3,5,7,9,....,99}
Now, a+b∈{25,71,117,163}
(i) a+b=25, no. of ordered pairs (a, b) is 12
(ii) a+b=71, no. of ordered pairs (a, b) is 35
(iii) a+b=117, no. of ordered pairs (a, b) is 42
(iv) a+b=163, no. of ordered pairs (a, b) is 19 ∴ total =108 pairs