Explanation

Rather than assigning separate variables to the granddaughters and grandsons, define them both in terms of the same unknown multiplier, based on the ratio given:

Number of granddaughters = 3m
Number of grandsons = m

Note that you are solving for 3m, not for m!

3m + m = 12

4m = 12

m = 3

3m = 9

Alternatively, suppose that Nash had exactly one grandson and three granddaughters.

That would sum to four grandchildren altogether. Triple the number of grandsons and granddaughters to triple the number of grandchildren.