the answer of question 1 is:
When given a bandwidth and a S/N ratio, you need to use the Shannon equation to determine the capacity of the channel in bits/second. Add 1 to your S/N ratio (25+1=26) and take the base 2 logarithm (Log2 of 26=4.7, that is, 2^4.7 = 26) and then multiply this result by the bandwidth (5000). In this case you'll get a capacity of 5000x4.7 = 23,500 bits/second.