A colony has 900 families. Of the 600 families in the older block, 70% own a two-wheeler, and across the whole colony 60% of the families own one. What per cent of the families in the newer block own a two-wheeler?

Older block owners = 70% of 600 = 420. Whole colony = 60% of 900 = 540. So the newer block accounts for 540 - 420 = 120 owners out of its 900 - 600 = 300 families: 120/300 = 0.40 = 40%. 50% is the trap of treating 60% as the midpoint of 70% and the unknown - that only works if the two blocks are the same size, and here the older block is twice as big, so it pulls the colony figure hard towards 70%. 20% divides the 120 by the older block's 600, and 13.33% divides it by all 900.