Question

A student score average 80 marks in 3 subjects physics chemistry and maths. if the average in physicsand maths is 90 and that in physics and chemistry 70. what are the mark in physics

Answer 1

$total \: marks = p + m + c= 80 \times 3 = 240 \\ marks \: of \: physics \: and \: maths = p + m = 2 \times 90 = 180 eq.1\\ marks \: of \: physics \: and \: chemistry = p + c = 2 \times 70 = 140 eq.2\\ adding \: eq.1 \: and \: 2 \\ 2p + c + m = 320 \\ p + c + m = 240 \\ substracting \: we \: get \\ p = 80$

Answer 2

Your answer :-
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Let the marks in Physics , Chemistry and Maths be p,c,m respectively.
then,
$\frac{p + c + m}{3} = 80$

$p + c + m = 240$

we know that ,

$\frac{p +m}{2} = 90$

$p + m = 180$
and ,

$\frac{p + c}{2} = 70$

$p + c = 140$
now we have
m = 180-p
c = 140 -p

putting values in main equation
$p + 180 - p + 140 - p = 240$
$- p = 240 - 320$

$- p = - 80$


$p = 80$

Hence , student got 80 marks in Physics .