jmc

We apply the distributive property repeatedly:

$$\begin{array}{rl}(x+10)(2y+10)& =x(2y+10)+10(2y+10)\\ & =x\cdot 2y+x\cdot 10+10\cdot 2y+10\cdot 10\\ & =\boxed{2xy+10x+20y+100}.\end{array}$$