### Unspecified function with some properties and substitution

This is an unformal question.

suppose that I have an undefined function $U()$ such that if $y\geq x$ then $U(y) \geq U(x)$.

I know that $U(16) = .2 U(10)+.8 U(20)$ .

I have to evaluate formaly

~~$EU_1=0.1~~*$EU_1=0.1 U(10) + *~~0.4~~U(16) 0.4U(16) + ~~0.5*U(20)$ ~~0.5U(20)$

where $EU_1$ is the expected utility of the lotery $l = [(10,0.1),(16,0.4),(20,0.5)]$. So first I want to substitute $U(16)$ in $EU_1$. On my sheet of paper this seems simple, but I do not know if I am able to write all this (with the substitution) in Sage ? Could some one help me.