Answer
a. 13.33 %
b. 33.33 %
c. 53.33 %
Work Step by Step
First, the volume is
$$1+2.5+4=7.5 \ parts$$
So the percent by volume in the dry mix is given by
a. cement:
$$
\frac{1}{7.5}\times 100 = 13.33 \ \%
.$$
b. sand:
$$
\frac{2.5}{7.5}\times 100 = 33.33 \ \%
.$$
c. gravel:
$$
\frac{4}{7.5}\times 100 = 53.33 \ \%
.$$
