T + 30*D = 153 (1) ("truck loaded with 30m^3 of rock has a total mass of 153t")
T + 22*D = 117 (2) {"truck loaded with 22m^3 of rock has a mass of 117t)
---------------------------Subtract eq(2) from eq(1) ==========>
8*D = 153 - 117 = 36 ==========> D = = 4.5 ton per m^3 is the density of the rock.
Then from eq(1), T = 153 - 30*D = 153 - 30*4.5 = 18 tons is the mass of the truck.