Chemical Kinetics and Nuclear Chemistry

Consider a reaction A+R⟶ Product.
The rate of this reaction is measured to be k[A][R]. At the start of the reaction, the concentration of R,[R]0, is 10-times the concentration of A,[A]0. The reaction can be considered to be a pseudo first order reaction with assumption that k[R]=k′ is constant. Due to this assumption, the relative error (in %) in the rate when this reaction is 40% complete is ____
[ k and k′ represent corresponding rate constants]