Step 1: According to the given data construct an imaginary figure for better understanding.

EXAMPLE:

Step 2: Make sure that all the measurements have the same units or not

NOTE: Convert the measurements into same units

EXAMPLE: Height of a girl = 90 cm

height of a lamp = 3.6 m = 3.6 * 100 = 360 cm

The girl walking speed = 1.2 m/sec = 120 cm/sec

Step 3: Find the distance travelled by the girl

NOTE: Distance = speed * time

EXAMPLE: From the figure

Distance (QT) = 120 \frac{cm}{sec} * 4 sec

QT = 480 cm

Step 4: Prove that the required triangles(PQR & STR) are similar

EXAMPLE: From the figure (PQR, STR)

\angle Q = \angle T = 90

And we have a common angle for two triangle that is

\angle R

So, PQR ~ STR (AAA similarity theorem)

Step 5: Use the AAA theorem to calculate unknown value

**AAA THEOREM:** In two triangles, if the angles are equal, then the sides

opposite to the equal angles are in the same ratio (or proportional) and hence

the two triangles are similar.

\frac{PQ}{QR} = \frac{ST}{TR}

Step 6: Find the length of the shadow

EXAMPLE: \frac{PQ}{QT + TR} = \frac{ST}{TR}

Substitute the known values

\frac{360}{480 + TR} = \frac{90}{TR}

360*TR = (480 + TR)* 90

270*TR = 480*90

TR = \frac{43200}{270}

TR = 160 cm

The length of the shadow = 160 cm