Step 1: Understand the question and underline the main points

NOTE: Refractive index μ = 1.5

First lens radius of curvature R_1= 25 cm

Second lens radius of curvature R_2 = 20 cm

The space between the lenses is filled with liquid of refractive index (μ) = \frac{4}{3}.

Step 2: Construct an imaginary figure by using the given data

We know that

\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} + \frac{1}{f_3}......................(1)

f = focal length

Step 3: Recall the lens makers formula

Know about lens makers formula

LINK: https://byjus.com/physics/derivation-of-lens-maker-formula/

FORMULA: \frac{1}{f} = ( μ - 1) [math] [\frac{1}{R_1} - \frac{1}{R_2}] [/math]

\frac{1}{f_1} = ( 1.5 - 1) [math] [\frac{1}{\infty} - \frac{1}{25}] = -12.5 [/math]

\frac{1}{f_2}= ( \frac{4}{3}. - 1) [math] [\frac{1}{25} - \frac{1}{-20}] =0.03 [/math]

\frac{1}{f_3} = ( 1.5 - 1) [math] [\frac{1}{-20} - \frac{1}{\infty}] = - 0.025 [/math]

From equation (1)

\frac{1}{F} = -12.5 + 0.03 + -0.025 = -12.495

F = 0.080