$78.90 = n(.05)+d(.10)

The below explanation can give you whatever you are seeking.

Step-by-step explanation:

The two unknowns in the equation are "n" and "d"

$78.90 = n(.05)+d(.10) ........................ (1)

Make both unknowns the subject of the formula and substitute for each other

............................ (2)

also

............................ (3)

You can substitute for either "n" or "d" in equation (1),

Putting equation (3) into (1)

78.90 = n(.05) +

78.90 = n(.05) + 78.90 - n( .05)

collecting all like terms

78.90 - 78.90 = n(.05) - n(.05)

0 = n(.05 - .05)

0/0 = n

n  is undefined

Putting equation (2) into (1)

78.90 =   + d(.10)

78.90 = 78.90 - d(.10) + d(.10)

78.90 - 78.90 = - d(.10) + d(.10)

0 = d (-.10 + .10)

0/0 = d

d is also undefined