book Maths Puzzle Numerical Ability

Linear systems of equations
x+y=11
x-y=10
where
x=
y=

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book Other Question

Can you please help to answer the following question:
.
Sum the series:
1 x 2 x 3 - 2^3 + 3 x 4 x 5 - 4^3 + ... + 9999 x 10000 x 10001 x - 10000^3
Two vats contains cream, one sample tested 8% butterfat and the other tested 16% butterfat. How much cream should be taken from each to produce a 5 liters mixture w/c will test 12%