Form 3 Maths 幫幫手 － Q 4

d) If Patrick walk 60cm backward...
Let new angle of elevation from Patrick's eye to Alfred's head be α, then
tan α = 18/(60+60) = 0.15
new distance y from Patrick's eye to Alfred's head at the ruler on the wall
tan α = y/(60+60+10)
y= 0.15 x 130 = 19.5
Therefore,
Patrick read about Alfred's head at the ruler on the wall = 138 + 19.5 = 157.5 =158cm (correct to nearest cm)