A standard flammability test for fabric is to place a standard-sized piece of fabric under a flame for a fixed period of time, and then measure the length of the charred portion of fabric. Suppose the procedure is carried out using a sample of 64 such portion of fabric and the average length of charred portion of fabric was 3 cm with a variance of 0.16cm. Construct a 90% confidence interval for the average length of charred portions of this fabric.