F.2 maths Rate and Ratio~急~20點

1.A car park charges $50 for the first 2 hours and $10 for each additional hour.Find the number of parking hours if the average parking rate is $ 16/h
Let x = number of hours
Total amount/ total hours = (2x$50 + 10x)/(2 +x) = (100 + 10x) / (2 +x)
This is also equal to $16/h
(100 + 10x)/(2+x) = 16
100 + 10x = 16(2 + x)
100 + 10x = 32 + 16x
6x = 100 – 32
6x = 68
x = 68/6 = 11 1/3
x =11 1/3 hours



2. The actual heights of three buildings A,B and C are 216m,336m and 402 m respectively. A model of each buiilding is constructed using the same scale. The height of the model of building A is 18 cm, what is the sum of the heights of the three models.
The scale 18 cm /216 m 18 cm / 21600 cm = 1 to 1200
For model B, 336 m/1200 = 0.28 m = 28 cm
For model C, 402 m/1200 = 0.335 m = 33.5 cm

Sum of the height of three models = 18 cm + 28 cm + 33.5 cm = 79.5 cm