Question

If xx and yy vary directly and yy is 15 when xx is 3, find yy when xx is 4.

Answer 1

Since \( xx \) and \( yy \) vary directly, we can express this relationship mathematically as:

\[
yy = k \cdot xx
\]

where \( k \) is a constant.

Given that \( yy = 15 \) when \( xx = 3 \), we can first find the value of \( k \):

\[
15 = k \cdot 3
\]

To find \( k \), we divide both sides by 3:

\[
k = \frac{15}{3} = 5
\]

Now we have the direct variation equation:

\[
yy = 5 \cdot xx
\]

Next, we want to find \( yy \) when \( xx = 4 \):

\[
yy = 5 \cdot 4 = 20
\]

Therefore, \( yy \) when \( xx \) is 4 is \( \boxed{20} \).