cmb Box, subform, and adding

[zyphac]

I have a form with 2 subforms. The first sub form is datasheet view, and one of my boxes is a cmb box. It selects type of rock: sand, rock, asphalt. Then my next subform is the totals. sand rock asphalt. I can't get them to add up say the total sand, aspfalt, rock. The subform in datasheet views data source is a query. Dumb me I forgot that there is a text box where you enter the weight(txtTons). Can I do this in the criteria of the query? I know how to add just the txttons:
=Forms!frmlogSheet!subfrmLogSheet.Form![Ton Total]
I have tried to find the right combination here, but this type of problem is new to me.
zypha