so... Piotr invested two amounts, say "a" and "b", at 3.5% and 4% respectively
whatever 3.5% of a is, and whatever 4% of b is, it ended up as 1440
now, we know that "b" amount is "$5000 more than three times" than "a" amount
so three times "a" is 3*a or 3a, now, 5000 more than that is 3a + 5000
now, assuming this is for a year alone,
how much is 3.5% of "a", well, 3/100 * a, or 0.035a
how much is 4% of "b", well, 4/100 * a, or 0.04b
so.. whatever those amounts yielded are, they ended up as 1440
so [tex]\bf \begin{cases}
\textit{3.5\% of a}\implies \frac{3.5}{100}\cdot a\implies 0.035a
\\\\
\textit{4\% of b}\implies \frac{4}{100}\cdot b\implies 0.04b\\
--------------\\
0.035a+0.04\boxed{b}=1440
\\\\
however\qquad b=\boxed{3a+5000}
\end{cases}[/tex]
do the substitution, and solve for "a", to see how much he invested at 3.5%
how much is "b"? well, b = 3a + 5000
