Find the interest amount.
[tex]I=1740-1200=540[/tex]
Use the formula for simple interest.
[tex]I=prt/100[/tex]
Solve for t.
[tex]540=1200 \times 15\% \times t[/tex]
[tex]540=1200 \times \frac{15}{100} \times t[/tex]
[tex]540=180t[/tex]
[tex]t=\frac{540}{180}[/tex]
[tex]t=3[/tex]