9.35 The abrasive resistance of rubber is increased by adding a silica filler and a coupling agent to chemically bond the filler to the rubber polymer chains.

Fifty specimens of rubber made with a type I coupling agent gave a mean resistance measure of 92, the variance of the measurements being 20.

Forty specimens of rubber made with a type II coupling agent gave a mean of 98 and a variance of 30 on resistance measurements. Estimate the true difference between mean resistances to abrasion in a 95% confidence interval.