What do you mean by 'monotonic preference' ?

Suppose there are two consumers in the market for a good and their demand functions are as follows:
d_$1$(p)= $20$-p for any price less than or equal to $20$, d_$1$(p)=$0$ at any price greater than $20$.
d_$2$(p)= $20$-p for any price less than or equal to $20$, d_$2$(p)=$0$ at any price greater than $15$.
Find out the market demand function.

Suppose there are $20$ consumers for a good and they have identical demand functions:
$d\left(\mathrm{p}\right)=10-3\mathrm{p}$ for any price less than or equal to $\frac{10}{3}$ and $d_1\left(\mathrm{p}\right)=0$ at any price greater than $\frac{10}{3}$.
What is the market demand function?