"My grandson is about as many days as my son is weeks, and my grandson is as many months as I am in years. My grandson, my son and I together are 100 years. Can you tell me my age in years?"
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let x = age of grandson in years
y = age of son in years
z = your age in years

no. of "x" age in days = no. of "y" age in weeks
x = y / 7 ; y =7x
no. of "x" age in months = no. of "z" age in years
x = z / 12 ; z =12x

then, x + y + z = 100
substitute y and z;
x + 7x + 12x = 100
or,
20x = 100
x = 5
y = y*x = 35
z = x*12 = 60 <--- your age!

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no more??