number of integral solutions of the equation

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Find the number of integral solutions of $\large 2x + y + z = 20$ with $x, y, z \ge 0$ ?

I saw a method of generating function but i donot have knowledge about generating function any alternative way to solve this , answer = 121

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$y+z=20-2x$ is one of $20,18,16,\cdots0$, so that there are $$21+19+17+\cdots1=121$$ solutions.