Why does t-butyl chloride undergoe solvolysis in 70% water/30% acetone at a rate slower than in 80% water/20% acetone?

The more polar solvent is better at stabilizing the developing carbocation.
The solvolysis of t-butyl chloride is an ##”S”_”N”1##reaction. It involves the formation of a 3° carbocation in a slow first step.
A polar solvent solvates and helps to stabilize the developing carbocation. This lowers the energy of the transition state and speeds up the reaction.
The greater the percentage of water in this solvent mixture, the more polar the solvent. The more polar the solvent, the better it is at solvating the developing carbocation and lowering the activation energy.
An 80:20 water/acetone mixture is more polar than a 70:30 mixture, so the solvolysis is faster in the more water-rich solvent.