(15*0.2)+(x*0.8)=100

Solution for (15*0.2)+(x*0.8)=100 equation:

Simplifying
(15 * 0.2) + (x * 0.8) = 100

Multiply 15 * 0.2
(3) + (x * 0.8) = 100
3 + (x * 0.8) = 100

Reorder the terms for easier multiplication:
3 + (0.8x) = 100

Solving
3 + (0.8x) = 100

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-3' to each side of the equation.
3 + -3 + (0.8x) = 100 + -3

Combine like terms: 3 + -3 = 0
0 + (0.8x) = 100 + -3
(0.8x) = 100 + -3

Combine like terms: 100 + -3 = 97
(0.8x) = 97

Divide each side by '0.8'.
x = 121.25

Simplifying
x = 121.25