Conversion Rate | Marketing Attribution

Hey, I just got done doing the Marketing Attribution Project, finding which campaign and source is successful when it comes to ads. Before step 7 we have the COUNT of each source with its campaign. When step 7 is completed, we know the COUNT of purchases through that source and campaign. How would I modify the code such that I could also have a column that shows the conversion rate for each source?

Here is the code:

WITH last_touch AS (
SELECT user_id,
MAX(timestamp) as last_touch_at
FROM page_visits
WHERE page_name = ‘4 - purchase’
GROUP BY user_id
),

lt_attr AS (
SELECT lt.user_id,
lt.last_touch_at,
pv.utm_source,
pv.utm_campaign
FROM last_touch AS ‘lt’
JOIN page_visits AS ‘pv’
ON lt.user_id = pv.user_id
AND lt.last_touch_at = pv.timestamp
)

SELECT lt_attr.utm_source,
lt_attr.utm_campaign,
COUNT(*)
FROM lt_attr
GROUP BY 2
ORDER BY 3 DESC;