At a florist, roses are sold at 4 stalks for $9 while carnations are sold at 7 stalks for $6. Mrs Ting bought an equal number of stalks of rose and carnations. She spent $117 more on the roses.
a) How many stalks of flowers did she buy altogether?
b) Her sister visited the same florist and spent an equal amount of money on the stalks of roses and carnations. What fraction of the stalks of flowers she bought were roses?
Source: Anglo-Chinese School Junior