a = hours worked by the first mechanic
b = hours worked by the second mechanic.
since the first mechanic charges $55 per hour, then for "a" hours that'd be a total of 55*a or 55a, likewise, for the second mechanic that'd be a total charge of 80*b or 80b.
we know all hours combined are 15, so then a + b = 15.
we also know that all charges combined are $950, so 55a + 80b = 950.