They're all simple.
150 = 25r
The r is multiplied for 25, so we should know what r equals to when it's not multiplied (1r).
So divide both terms for 25.
150/25 = 25r/25
6 = r
r = 6 <- answer
38 = 4p
Same thing here (divide both terms for 4)
38/4 = 4p/4
38/4 = p
p = 38/4 and simplify p = 19/2 <- answer
18 = a/20
We have a fraction, and we have to "delete" it.
There are two ways.
First one: multiply both terms for the denominator of the fraction.
18*20 = a/20 * 20
360 = a
a = 360 <- answer
Second way.
Take the two fractions to the common denominator (20) and do the LCM.
360/20 = a/20. Multiply both for 20
360 = a
a = 360
0 = 10x
0/10 = 10x/10
x = 0 <- answer