Question

what number should be added to each of the numbers 12 ,22, 42 and 72 so that the resulting numbers may be in proportion

Answer 1

let the number to be added be X.
therefore:
$12 + x \div 22 + x = 42 + x \div 72 + x$
cross multiply and create an equation. The following will be formed.

$(72 + x) \times (12 + x) = (22 + x) \times (42 + x)$
on solving it further you will get:

$20x = 60$
therefore the number will be 3.