Hi Kane535,

this is a binomial situation in which a calculator breaks down within 2 years or doesn't.

where p=probability of a breakdown in 2 years, which is 0.25

q=probability the calculator is still functional after 2 years, which is 1-0.25=0.75.

is the probability of having exactly n functional calculators after 2 years.

(a)

(b)

At most 3 is 3 or less, which is 0, 1, 2 or 3 breakdowns

(c)

At least 3 is 3, 4, 5 or 6 breakdowns

(d)

Either 2 or 3