Answer:
x = 
Step-by-step explanation:
Let the number be x
x + 2x² = 10
Factorize x + 2x²
x(1 + 2x) = 10
Divide both sides by (1 + 2x)
x = 
Rational ! the number doesn't repeat forever or anything like that. it simplifies nicely.
Yes, y=-9x is proportional
The first one is 20 and the second one is 19 and the third one is 25 and the fourth one is 15 and the fifth one is 11 and the sixth one is 28
5,000 × 3.8%= 19,000×18= 342,000 × 40 = 13,680,000