P5/P6 Math: Partially-filled Container becomes Lighter

Question like these gives a total mass of a container that is partially filled. (fraction)
The mass given includes the contents of the container and also the mass of the container itself.

Such questions are common in Paper 2 of P5 and P6 exams.
Calculator usage is accepted.

More of such questions (of varying difficulty) can also be found here.

A glass container, when 7/8 filled with pebbles, has a mass of 1780g.
After 243g of pebbles were poured out, 
the glass container is now half filled with pebbles.

What is the mass of the glass container?



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