Math 6 High ( Period 3)

 Finding a Percent of a Number 7.2

or… it’s just proportions!

Percent/100 = Percent of a number/number

Find 30% of 400
30/100 = x/400
Its a proportion!
30(400) = 100(x)
30(400)/100 = x
120 = x
Therefore 120 is 30% of 400

You drive 300 miles. the first 20% of the road is under construction. How many miles of the road are under construction?
20/100 = x/300
Its a Proportion!!
20(300) = 100(x)
20(300)/100 = x
60= x
therefore 60 miles of the road are under construction.

You can also change the percent to a fraction and multiply!
For example,
Find 50% of 46. 
[Remember: the word "OF" in a mathematical sentence means to multiply]

50% is just 1/2 
so
1/2(46) = 23

75% of 120
75% = 3/4
so 3/4 (120) = 90

You can also change the percent into a decimal and multiply
Find 35% of 150
Read this as
What is 35% of 150?
x= 0.35(150)
x = 52.5
Find 92% of 75
Read this as
What is 92% of 75?
x = 0.92(75)
x = 69

Are these reasonable?
Well 92% is pretty close to 90% and !0% of 75 would be 7.5 so 90% would be 75-7.5 or 67.5.   Since  69 is very close to 67.5  the answer is reasonable.