Interpret the given quantities

For a convex lens, let:

• actual object distance from the lens be $u$
• actual image distance from the lens be $v$
• focal length be $f$

The question says that the distances are measured from the focus.

So define:

• object distance from focus: $u'$
• image distance from focus: $v'$

Then, $u = f + u'$ $v = f + v'$

Use Newton’s form of the lens formula

For a thin lens, when distances are measured from the focal points, Newton’s formula is: $u'v' = f^2$

The graph/equation given in the problem is: $u'v' = 225$

Comparing with Newton’s formula, $f^2 = 225$

Calculate the focal length

$f = \sqrt{225} = 15\ \text{cm}$

Since magnitude is asked, $|f| = 15\ \text{cm}$

Compare with stored answer

Derived answer: $15$

Stored correct answer: $15$

Hence, they agree.