Answer
See the answer below.
Work Step by Step
a) From the rate law $r=k[PH_3]$, we deduce it's a first-order reaction, for which:
$\ln(C/C_0)=-0.693/t_{1/2}\times t$
Decomposing 3/4 means that $C/C_0=0.25$
$\ln 0.25=-0.693/37.9\ s\times t$
$t=75.8\ s$
b) $\ln (C/C_0) = -0.693\times 60\ s/37.9\ s$
$(C/C_0)=0.334\ or\ 33.4\%$
The remaining fraction is 33.4% of the original.
