The chance of raining tomorrow (B) is 70% while the chance of having a windy (A) and rainy weather is 50%. The chance hat it is windy, given that it is rainy is obtained from the rule of probability: p(A/B) = p (A and B) / p (B)
substituting,
p(A/B) = 0.50/0.70 equal to 0.7143