A triangular sticker is redrawn as a similar smaller sticker whose area is 64 per cent of the original area.
By what percentage is each side reduced?

The length factor is $\sqrt{0.64}=0.8$, so each side becomes 80 per cent of its old length, a reduction of 20 per cent. The option 36 per cent is the reduction in AREA (100 - 64), 80 per cent quotes the fraction of each side that SURVIVES rather than the amount removed, and 8 per cent misreads the factor 0.8 as a percentage.