smc

We have that $12=\frac{60}{5}$. We can substitute our value for 5, to get $$12=\frac{60}{60^b}=60\cdot 60^{-b}=60^{1-b}.$$ Hence $$12^{(1-a-b)/\left(2\left(1-b\right)\right)}=60^{(1-b)(1-a-b)/\left(2\left(1-b\right)\right)}=60^{(1-a-b)/2}.$$ Since $4=\frac{60}{3\cdot 5}$, we have $$4=\frac{60}{60^{a}60^b}=60^{1-a-b}.$$ Therefore, we have $$60^{(1-a-b)/2}=4^{1/2}=\boxed{2}$$