Question

420 mens manufacturer building in 9 months how many men will be required to manufacture a building in 7 months

Answer 1

the above question is an excellent example of inverse variation
420×9÷7=540men

Answer 2

Work and Time is always Inverse Variation.
$x ^{1} y ^{1} = x^{2} y {}^{2} \\ = > 420 \times 9 \: months \: = x \times 7 \: months \\ = > (420 \times 9) \div 7 = x \\ = > 60 \times 9 = x \\ = > x = 540$
540 men are required