$-$ Problem $5($Revelle $15-25)$

A construction firm buys and replaces its dump trucks periodically. It is considering the choice between two versions of the same make of truck: $1)$ the standard version, which costs $\backslash (\backslash$$ $50,000\backslash )$ and is replaced every $7$ years or $2)$ the heavy$-$duty version, which costs $\backslash (\backslash$$ $70,000\backslash )$ and whose replacement frequency is unknown to the firm. If operation and maintenance costs of the two trucks are assumed to be the same, how long must the heavy$-$duty truck last in order to be equivalent to the standard version? Solve using annual cash flow analysis and draw the cash flow diagrams and break even chart. The firm uses a MARR of $\backslash (10\backslash \%\backslash ).$