Show me step by step how to solve 6/13 =3/c ?

6/13 = 3/c
You need to cross multiply to solve this.
That means you need to multiply the numerator on the left by the denominator on the right and the denominator on the left by the numerator on the right.

6c = 39
c = 6.5

To check it, input the number you got into the c in the equation.

6/13 = 3/6.5
0.46 = 0.46

On monday,mrs.wise bought 3 lbs of apples at $4per lb= 4*3=$12on tuesday ,20%off and she buy 5 lbs of apples = $4-20%=3.20*5lb=$16example: other wise when she buy 5lbs on monday she have to spend 4*5=20

B. because the 7 represents "seven times", x represents "a number", 12 represents "plus twelve", and -4 represents "minus four"