What does it mean by C 1 T 1 +C 2 T 2 +C 3 T 3 +C 4 T 4/ 8 ?

What does it mean by C1T1+C2T2+C3T3+C4T4/8? The numbers are supposed to tiny and to the right of the big Letter. I don't remember what that's called. Are they referring to the corresponding bits of information I have. They aren't clearly labeled as such, except as time period and 4 time periods and concentrations and have 4 corresponding listings in ppm.