Question

M minutes less than 80 minutes is 15 minutes

Answer 1

"80 minutes" . . . . . . . . . . . . . .            80

"M minutes less" . . . . . . . . . . .            80 - M

"... is  5 minutes" . . . . . . . . .                80 - M   =  15

Add 'M' to each side . . . . . . .              80          =  15 + M 

Subtract  15  from each side               80 - 15  =  M

Do the subtraction on the left side:     65          =  M

Answer 2

In this problem we know that total minutes are 80 and
80 - M = 15
let's solve by steps 

Step 1 - adding M on both sides
80 - M + M = 15 + M
80 = 15 + M

Step 2 - subtract 15 from both sides
80 -  15 = 15 - 15 + M
65 = M

So, 80 - 65 = 15 
Hence verified.

Hope this helps you :)