M = Music CDs

C = cassettes

Create two equations, one for the number of music purchases he made (14) and one for the price of the music purchases ($112)

M + C = 14 ( this is the number of music purchases he made)

10M + 3C = 112 ( this is the price of the music purchases)

Isolate a variable, I isolated M

M + C = 14

M = 14 - C

Subsitute that variable into the other equation

10 M + 3C = 112

10(14-C) + 3C = 112 ( substitute our previous equation into M)

140 - 10C + 3C = 112

-10C + 3C = -28

-7C = -28

C = 4

Plug C back into either equation, I chose the simpler one

M + C = 14

M + (4) = 14

M = 10

Since he bought 4 cassettes for $3 dollars each 4($3) = $12 worth of cassettes.

Since he bought 10 Music CDs for $10 dollars each 10($10) = $100 worth of cassettes.

C = cassettes

Create two equations, one for the number of music purchases he made (14) and one for the price of the music purchases ($112)

M + C = 14 ( this is the number of music purchases he made)

10M + 3C = 112 ( this is the price of the music purchases)

Isolate a variable, I isolated M

M + C = 14

M = 14 - C

Subsitute that variable into the other equation

10 M + 3C = 112

10(14-C) + 3C = 112 ( substitute our previous equation into M)

140 - 10C + 3C = 112

-10C + 3C = -28

-7C = -28

C = 4

Plug C back into either equation, I chose the simpler one

M + C = 14

M + (4) = 14

M = 10

Since he bought 4 cassettes for $3 dollars each 4($3) = $12 worth of cassettes.

Since he bought 10 Music CDs for $10 dollars each 10($10) = $100 worth of cassettes.