A radioactive material has half life time of 10min. The time interval between the 40% decay to 85% decay.

For 40% decay
$\begin{array}{l}\frac{\mathrm{N}}{{\mathrm{N}}_0}=\frac{1}{2^{{\mathrm{n}}_1}}\Rightarrow \frac{60}{100}=\frac{1}{2^{{\mathrm{n}}_1}}------\left(1\right)\\ {\mathrm{n}}_1=\frac{{\mathrm{t}}_1}{\mathrm{T}};\text{ For }85\mathrm{\%}\text{ decay }\\ \frac{\mathrm{N}}{{\mathrm{N}}_0}=\frac{1}{2^{{\mathrm{n}}_2}}\Rightarrow \frac{15}{100}=\frac{1}{2^{{\mathrm{n}}_2}}-----\left(2\right);{\mathrm{n}}_2=\frac{{\mathrm{t}}_2}{\mathrm{T}}\\ \frac{\left(2\right)}{\left(1\right)}\Rightarrow \frac{15}{\frac{100}{60}}=\frac{1}{2^2}=\frac{1}{2^{\left({\mathrm{n}}_2-{\mathrm{n}}_1\right)}}\Rightarrow \frac{1}{2^{\left(\frac{{\mathrm{t}}_2-{\mathrm{t}}_1}{\mathrm{T}}\right)}}\end{array}$
t₂ – t₁ = 2×T = 2×10 = 20min