... 0.001¹

If $20\log_{10}(a) = 60$ dB, then $\log_{10}(a) = 60/20$, and taking each side to a power of 10 yields $a = 10^3 = .001$.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.