Hi

Let X be the random variable that counts the number oh houses that he sells per week.

Let Y be the random variable of his pay.

He is paid 100£ + 50£ for each house sold.

The expected value of this pay is

100+50k)\:\binom{50}{k}\:0.2^k\:0.8^{50-k}" alt="\sum_{k=0}^{50}\100+50k)\:\binom{50}{k}\:0.2^k\:0.8^{50-k}" />

which you can find equal to 600£ by splitting the sum into 2 parts.