T=\frac{2.303}{K} \mathrm{log} [\frac{100}{100-x%}]
{T}_{75%}=\frac{2.303}{K} \mathrm{log} [\frac{100}{25}]=90
{T}_{60%}=\frac{2.303}{K} \mathrm{log} [\frac{100}{40}]
\frac{{T}_{75%}}{{T}_{60%}}=\frac{2\mathrm{log}2}{\mathrm{log}2.5} ⇒ \frac{90}{{T}_{60%}}=\frac{2\times 0.3}{0.4}
{T}_{60%}=\frac{90\times 4}{6}=60 \mathrm{min}.