[jAc]

In[1]:=
data = {{1, 109.78}, {2, 80.35}, {3, 58.81}, {4, 43.04}, {6,
23.05}, {8, 12.35}, {10, 6.61}};
{a, b} = {a, b} /. FindFit[data, a*Exp[-b*x], {a, b}, x]
func = a*Exp[-b*x]
c = a + b

Out[62]= {150., 0.31213}

Out[63]= 150. \[ExponentialE]^(-0.31213 x)

Out[64]= 150.312