Griffith & Associates Is Trying to Determine How Often It

Griffith & Associates is trying to determine how often it needs to deposit payroll taxes for the calendar year 2017.The company made the following quarterly payroll tax deposits during the last two years:
$\begin{array}{lr}\text {Quarter beginning January 1, 2015 }&\$11,000\\\text {Quarter beginning April 1, 2015 }&11,000\\\text { Quarter beginning July 1, 2015}&10,000\\\text {Quarter beginning October 1,2015 }&10,000\\\text {Quarter beginning January 1, 2016 }&15,000\\\text {Quarter beginning April 1, 2016 }&16,000\\\text {Quarter beginning July 1, 2016 }&12,000\\\text {Quarter beginning October 1,2016 }&18,000\\\end{array}$
a.What is the lookback period and amount?
b.In 2017,how often must Griffith & Associates make payroll deposits?