Question 404935
J =Joel
J+25 = Father
J+51 = Grandfather 

we know that J+F+G=100 => J + J + 25 + J + 51 = 100 => 3J + 76=100 => 3J=100-76
=> 3J = 24 => J = 24/3 = 8
Joel is 8 years old
Joel's father is 8 + 25 = 33y.o
Joel's Grandfather is 8 + 51 = 59y.o

Verification
8 + 33 + 59 = 100