Answer :
1st piece = x
2nd piece = 2x
3rd piece = 7x + 1
so x + 2x + 7x + 1 =51
10x +1 = 51
10x = 50
x = 5
so 1st piece = 5
2nd piece = 10
3rd piece = 36
2nd piece = 2x
3rd piece = 7x + 1
so x + 2x + 7x + 1 =51
10x +1 = 51
10x = 50
x = 5
so 1st piece = 5
2nd piece = 10
3rd piece = 36