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"?
[tex]\bf A\ \textless \ B\implies 90+4t \ \textless \ 70 + 10t\implies 90-70 \ \textless \ 10t-4t
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20 \ \textless \ 6t\implies \cfrac{20}{6}\ \textless \ t\implies \cfrac{10}{3}\ \textless \ t\implies 3\frac{1}{3}\ \textless \ t
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\textit{3 years and }\frac{1}{3}\textit{ or \underline{3 years and 4 months}}[/tex]
after that period, then A becomes cheaper than B.
