Which expression represents 'half of the sum of aaa and bbb, decreased by 10'?
a+b2−10\frac{a+b}{2} - 102a+b−10
a+b−102\frac{a+b-10}{2}2a+b−10
12(a+b−10)\frac{1}{2}(a+b-10)21(a+b−10)
2(a+b)−102(a+b) - 102(a+b)−10