Total change in grade,

*A*:

$A = g_1 - g_2$

$A = 0.80 - (-0.40)$

$A = 1.20\%$

Number of meter-stations, *n*:

$n = \dfrac{\text{total change in grade, }A}{\text{change in grade per meter-station}}$

$n = \dfrac{1.20\%}{0.20\%}$

$n = 6 ~ \text{stations}$

Length of vertical curve, *L*:

$L = \text{number of meter-stations } ~ \times ~ \text{ length of a meter-station}$

$L = 6 \times 20$

$L = 120 ~ \text{m}$ [ C ] *answer*