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In 30 minutes, the truck weighs 33 tons, which means the mulch is 18 tons (33 tons -15 tons from the truck).
In 45 minutes (15 minutes later), the truck weighs 42 tons, which means the mulch is 27 tons.
So from 30 to 45 minutes, 15 minutes elapsed. And in those 15 minutes, 9 tons of mulch was loaded into the truck (27 tons of mulch - 18 tons of mulch).
This can be expressed as
In 45 minutes (15 minutes later), the truck weighs 42 tons, which means the mulch is 27 tons.
So from 30 to 45 minutes, 15 minutes elapsed. And in those 15 minutes, 9 tons of mulch was loaded into the truck (27 tons of mulch - 18 tons of mulch).
This can be expressed as