The Basic Practice of Statistics 7th Edition

Marginal distribution is the sum of counts of each column divided by sample size of 1808 As & Bs = $\frac{736+205}{1808} = 0.5205 = 52.05%$ Cs = $\frac{450+144}{1808} = 0.3285 = 32.85%$ Ds & Fs = $\frac{193 + 80}{1808} = 0.1510 = 15.10%$ From this we can deduce that 32.85% + 15.10% = 47.95% of students received a Grade C or lower