Want to do 140 pushups a week?
Easy. Just do a 20 a day for 7 days.
Want to do 280 pushups a week?
Easy. Just make the 20 you do per day four count pushups.
Want to have sore arms, back and chest?
Easy. Just follow either one of the suggestions above.
There is more than one type of pushup. If you haven’t done one in a long time start by using the wall. Stand far enough from the wall that your arms are fully extended straight ahead and your palms are on the wall while you stand straight up. Lean in and push out while keeping your back straight. Do your best not to bend at the waist.
There’s no better way to build your pecs and arms, and it is relatively safe, though you can end up with a sore shoulder if you overdo it.