Jason's gross monthly income is $$3800$ a month and he pays monthly expenses of $$670$ for his mortgage payment, $$105$ for heating, and $$187$ in property taxes.

a$)$ Determine Jason's Gross Debt Service Ratio $($GDSR$)$ if he decides to make monthly mortgage payments of $$900$ instead of $$670.$

b$)$ Based on the GDSR can Jason afford to make these payments? Justify your answer.