additive variations

evening all
I am a little bit stuck.
i am doing the bpp book for additive variations.
I am working my way through - but this bit i cannot work out.
i have attached a file to this message.
i dont understand how the seasonal variation is worked out. it has given me the formula Y - T but no matter how i work it, i do not get to the figures in the final column.
i thought i could figure this out, but been looking at it for an hour now and am stuck.
Hope some one can help - i am sure they can
Many thanks
Tracy
I am a little bit stuck.
i am doing the bpp book for additive variations.
I am working my way through - but this bit i cannot work out.
i have attached a file to this message.
i dont understand how the seasonal variation is worked out. it has given me the formula Y - T but no matter how i work it, i do not get to the figures in the final column.
i thought i could figure this out, but been looking at it for an hour now and am stuck.
Hope some one can help - i am sure they can
Many thanks
Tracy
Comments
((600+840+420+720)/4)+((840+420+720+640)/4)/2
the actual sales in Yr 5 Q3 was 420.
Actual - Trend = seasonal variation
420 - 650 = -230
thanks
Tracy