In the following Venn diagram, the triangle stands for 'fathers', ', the hexagon stands for 'tax-payers', and the rectangle stands for 'blood donors'. The given numbers represent the number of persons in that particular category.

How many tax-payers are also blood donors?

63

The triangle stands for 'fathers'

The circle stands for 'engineers

The hexagon stands for 'tax-payers

The rectangle stands for 'blood donors'. 

Step 1 ) Look at the figure of tax payers and blood donors ( hexagon and rectangle)

Step 2) Look at common number between them that is 27 and 36

Step 3) Add both numbers and get the answers easily (27 + 36) = 63