Let a man do \(\frac{1}{x}\) work per day and
each woman do \(\frac{1}{y}\) work per day
\(\frac{12}{x} + \frac{16}{y} = \frac{1}{5}............\)(1)
\(\frac{13}{x} +\frac{24}{y} = \frac{1}{4}\) ............ (2)
Multiply (1) by 3 & (2) by 2
\(\frac{36}{x} + \frac{48}{y} = \frac{3}{5}\) ....... (3)
\(\frac{26}{x} + \frac{48}{y} = \frac{1}{2}\) ........ (4)
Equation (3) - (4)
\(\frac{10}{x} = \frac{1}{10}\)
x = 100
y = 200
We require to find z; substituting the values
\(\frac{7}{x}+\frac{10}{y}= \frac{1}{z}\)
\(\frac{7}{100}+ \frac{10}{200} = \frac{1}{z}\)
\(\frac{7}{100}+ \frac{5}{100} = \frac{1}{z}\)
\(\frac{12}{100} = \frac{1}{z}\)
\(z = \frac{100}{12} = 8.33 = Answer\)
This is the most straightforward way and it is what I did but it is way too time-consuming, it is better to check the choices first and see if they are far apart, if they are, it is better to solve it with shortcut methods mentioned by others.