Two types of coins are being tossed

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Let's assume we have two types of unfair coins. One has $70\%$ probability of getting head, and the other has $70\%$ change of getting tail. Now if we throw coin A $k$ times and coin B $n$ times. What will be the probability of getting at least $w$ heads. $w < k+n.$ I have it figured out when it comes to one coin. By using Normal distribution (aproximating Binomial distribution). But things get too complicated with second coin. Could you help me out?

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Let $X$ be the number of heads for coin $A$, and $Y$ the number for coin $B$. $X$ is approximately distributed like $N(0.7k, 0.21k)$, while $Y$ is approximately $N(0.3n, 0.21n)$. This means that $X+Y$ is approximately $$ N(0.7k+0.3n,0.21(k+n)), $$ so you can use a normal distribution with these parameters to approximate $X+Y$.