Aconstruction firm is trying to decide which of two models of a crane to purchase. model a costs $90,000 and requires $4,000 per year to maintain. model b has an initial cost of $70,000 and a maintenance cost of $10,000 per year. for how many years must model a be used before it becomes more economical than b?

Let's say hmm let's use the figures in thousands, so
model A costs 90 + 4 per year, so... ok.. after "t" years, then the cost for model A is then 90 + 4*t, or A = 90 + 4t.

now, model B is 70 + 10 per year, so,... after "t" years, model B is then
B = 70 + 10 * t, or B = 70 + 10t.

if we were to assume A < B, namely the cost of A is less than B, what would the years be? namely, what's "t"?



after that period, then A becomes cheaper than B.