Question

find two decimal fraction whose sum is 0.9 and deffrence is 0.2

Answer 1

Let a and b be the 2 numbers:
a + b = 3.9
a - b = 0.9
a*b = 3.6
Lets solve for a in 2nd equation:
a+-+b+=+0.9
Add b on both sides
a+=+b+%2B+0.9
Now substitute this in for a in 1st equation:
%28b%2B0.9%29+%2B+b+=+3.9
Add like terms
2b+%2B+0.9+=+3.9
Subtract 0.9 on both sides
2b+=+3
Divide by 2 on both sides
b+=+1.5
Substitute this value in for b to solve for a:
a+=+b+%2B+0.9
a+=+1.5+%2B+0.9
a+=+2.4
Therefore the 2 numbers are 1.5 and 2.4
You can check that 1.5*2.4 = 3.6

Answer 2

x+y=0.9
x-y=0.2
adding the equations,
2x=1.1
x=1.1/2=11/20=0.55