Question

What is the value of $x$ for the following equations: $x-5y=10$ and $x+y=4$?

(Use cross multiplication method).

• A. $x=5$
• B. $x=4$
• C. $x=3$
• D. $x=2$

Answer 1

Answer: (A)

$x=5$

$x-5y=1$0---(1)
$x+y=4$---(2)
Using formula for cross multiplication method:
$\frac{x}{(b_1{c}_2-b_2{c}_1)}=\frac{y}{(c_1{a}_2-a_1{c}_2)}=\frac{-1}{(a_1{b}_2-a_2{b}_1)}$
So, from equation (1) and (2) we can write the value of $a,b$ and $c$.
$\frac{x}{-5\times 4-1\times 10}=\frac{y}{10\times 1-1\times 4}=\frac{-1}{1\times 1-1\times (-5)}$
$\frac{x}{-20-10}=\frac{y}{10-4}=\frac{-1}{1+5}$
$\frac{x}{-30}=\frac{y}{6}=\frac{-1}{6}$
$\frac{x}{-30}=\frac{-1}{6}$
$6x=30$
$x=5$