First convert all the fractions so that they have a common denominator.
1/2= 8/16
1/8= 2/16 u multiply this by 2 because he is giving 2/16 to each of his 2 children so it’s 4/16
U then multiply 1/16 by 3 because he is giving 1/16 to each of his 3 grandchildren so 3/16 in total
Then add up all these fractions 8/16+4/16+3/16= 15/16
U know that he’s giving away the remaining 15,000 which is 16/16-15/16 so 1/16 of his total estate. Then u can set a proportion to find out his total estate 1/16=15,000/x and u get that x=$240,000