For computing this, follow this method:
4 loads of stone weigh 2/3 ton.
4loads---------------------2/3
1load---------------------?
So it will be computed as 1x 2/3 /4 = 2/3x4= 1/3x2=1/6
the weight of 1 load of stone is 1/6 ton
Hope this helps. Let me know if you need additional help!