Luke has $21 more than Rachel and $48 more than Daniel. All together they have $

168. How much money does Luke have?

This can be solve by 3 variable equation

let x be the money of luke

y money of rachel

z money of daniel

first equation

x = y + 21

second equation

x = z + 48

third equation

x + y + z = 168

solving simultaneously

x = 79

y = 58

z = 31