What is the estimate answer and the exact answer of 17/10 ÷2 4/5

Both 17/10 and 2 4/5 should be rounded up in preparation for this estimation.

17/10 is close to 2 and 2 4/5 is close to 3.  Thus, your estimated answer should be
                     2/3, or approx. 0.6666

Exact    divide 17/10 by 14/5.  LCD is 10, so convert 14/5 to 28/10.

Now divide 17/10 by 28/10.    17/28 = approx.   =  0.607 approx.

These two results are comparable:   0.6666  and 0.6071

B. 2 1/2 feet. multiply 2.5 inches by 12 to get thirty inches then divide by 12 to get 2.5 feet