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The simple way to do this is to think about what the facts mean.
Each truck that comes up to the loading bay would reduce the amount by 840 units.
Therefore to find the number of trucks divide the total amount by the number of unit each truck can carry as this is the number of times 840 units fits into the total.
19420/840 = 23.12
However you can have 23 trucks and then a part of the truck to carry the extra 0.12 units. You would have to still use a whole truck to carry this extra so this means 23 + 1 truck = 24 trucks.