A movie producer earns 5 million every time he wins an award at a film festival, but loses 2 million every time he fails to win. He has won 70 million after 21 festivals. How many awards has he won ?
The easiest way is trial and improvement/iteration. Try something you think is close- e.g. won 12 lost 9 and see how close you are (60m less 18m= 42m.) This is too low so try something higher such as won 18 lost 3 (90m-6m=84 m: too high). So go somewhere in between until you get there.
Or you can solve 2 simultaneous equations: 5W- 2F= 70 and W+F= 21.
The second can be rewritten as 2W +2F=42. So 7W= 122, so W = 16