Let x represent the number of books that Tyrone has.
Let y represent the number of books that Lei has.
Given that Tyrone has four times as many books as Lei, it means that
x = 4y
Together they have 50 books. It means that
x + y = 50
Substituting x = 4y into x + y = 50, it becomes
4y + y = 50
5y = 50
y = 50/5
y = 10
x = 4y = 4 * 10
x = 40
Tyrone has 40 books
Lei has 10 books