For X we take the central datum as representative of the range, so, for example, 10-20 would be represented by 15. The sum of the data is 7×15+3×25+6×35+10×45+15×55=5(21+15+42+90+165)=1665.
The number of observations=7+3+6+10+15=41, so the mean, μ, is 1665/41=40.61 approx.
To calculate the standard deviation, we first need the variance, σ2:
{7(15-μ)2+3(25-μ)2+6(35-μ)2+10(45-μ)2+15(55-μ)2}/41=214.8721, standard deviation, σ=14.66 approx.
Coefficient of variation=σ/μ=0.36 approx.