$\dfrac{6}{3+4+5+6}\times360^{\circ}-\dfrac{3}{3+4+5+6}\times360^{\circ}=\dfrac{360}{18}\left( 6-3\right)=20\times3=60^{\circ}$ 
If the degree measures of the angles of a convex quadrilateral are in the ratio~$3:4:5:6$, by how many degrees does the measure of the largest angle exceed the measure of the smallest angle? 
Options:
* $30^{\circ}$ 
* $45^{\circ}$ 
* $60^{\circ}$ 
* $75^{\circ}$ 
* $90^{\circ}$ 