The exact tonnage depends on the density of rock. Whereas loose drain rock material weighs less than 3000 lb/yd3, compacted base rock can weigh around 4000 lbs/yd3.
Assuming you are referring to drain crushed rock, the weight of 158 cubic yards would be 158 x 3,000 = 474,000 lbs which is 237 tons.
And to convert the 30 tons to cubic yards for the same material: 30 tons = 60,000 lbs. And the volume would be 60.000/3,000 = 20 cubic yards.
Hope this helps.