The objective of the game Minesweeper is to clear a field without detonating any mines or bombs. Your friend claims that his rate of completing the game successfully is \(90 \%\).

a. You decide to challenge your friend. He makes 10 attempts to complete the game but is successful only 7 times. Does this mean that your friend's claim is wrong?

b. If your friend's claim was true, what would you have to do to ensure that the cumulative proportion of his successful attempts to complete the game falls very close to 0.9?