Lucy has some number cards that each have a whole number on them. If she chooses a card at random, the probability that the number on it is even is $\frac{3}{7}$ Work out the ratio of even cards to odd cards that Lucy has. Give your answer in its simplest form.

Step 1 ：We are given the probability of choosing an even card as \(\frac{3}{7}\). Let E be the number of even cards and O be the number of odd cards. We can write the probability as a fraction: \(\frac{E}{E + O} = \frac{3}{7}\)

Step 2 ：Solve the equation for E in terms of O: \(E = \frac{3}{4}O\). The simplest ratio of even cards to odd cards is \(\boxed{3:4}\)