Optical purity = $\frac{\mathrm{Observed}\mathrm{specific}\mathrm{rotation}}{\mathrm{specific}\mathrm{rotation}\mathrm{of}\mathrm{pure}\mathrm{enantiomer}}$ × 100
optical purity of d-isomer = $\frac{6.76}{13.52}\times 100$ = 50%
optical purity + racemic mix = 100
$\therefore$ racemic mixture = 100 – 50 = 50%
$\therefore$ % d isomer = 25%
    & % l isomer = 25%   
$\therefore$ % composition of d-isomer = (50 + 25) % = 75%