If 0.5x + 0.7y=0.5 and 0.7x + 0.5y=0.7,then the value of x+y is

12 × 15 + 27 – ? = 28% of 400

If (x +y) = 6 and x² + y² = 20 then find the value of (x - y) ².

How many perfect squares lie between 120 and 300? 

Simplify the following equation.
$${(785.785.785+435.435.435) \over(785)^{2}+(435)^{2}-(785)(435)}$$

If 2x + 3y - 5z = 18 , 3x + 2y + z = 29 and x + y + 3z = 17 , then what is the value of xy + yz + zx ? 

[(169×121)÷(11×13)]-3=?

If 3x + 7y=75, 5x- 5y= 25, then the value of (x +y)=?