The boker has already made 40 trades and 80% of them are profitable.

Let us suppose that the broker makes $x$ more trades and looses them all.

Now this value of x is such that his percent of profitable trades become 50%.


Initially the broker had made: 40 trades of which 80% were profitable or $\frac{80}{100} \times 40=32$ profitable and 8 loss making.

Now with the new $x$ number of trades he is 50% loss making so $\frac{50}{100} \times (40+x)=8+x$.

Remember that he already had 8 trades which lost money previously. Solve for x and you shall get the value $x=24$.

So if he makes 24 more trades and losses them all he will have 50% profitability.


You can cross check by putting the values

40 trades ---------------- 32 profit ---------------- 8 loss

40 +24 trades ------------ 32 profit ---------------- 8 loss +24 loss =32 loss.

After 24 the profitability is 50% not BELOW 50%.

He needs to loose 25 trades to go below 50%. Hence option B is correct!