Answer-<p>What is the total number of residents in the building?</p>

Let the number of people who read lsquoThe Hindu and TOI Onlyrsquo be x Let the number of people who read lsquoThe Hindu and Economic Times Onlyrsquo be y. Let the number of people who read lsquoTOI and Economics Times Onlyrsquo be z. Given that 40 20 x y 100 gtxy 40 ---- i 70 20 xz 150 gtxz 60 ----ii 100 20 yz 200 gtyz 80 -----iii Adding eq. i, ii and iii, we get 2 xyz 180 xyx 90 ----iv Now, subtracting iv by i, we get z 50 Now, subtracting iv by ii, we get y 30 Now, by subtracting iv by iii, we get X10 Total number of residents in the building 40 70 100 10 50 30 20 320.