Answer:
r = 9
Step-by-step explanation:
Given that c varies directly as (r + 1) then the equation relating them is
c = k(r + 1) ← k is the constant of variation
To find k use the condition c = 8 when r = 3, then
8 = k(3 + 1) = 4k ( divide both sides by 4 )
2 = k
c = 2(r + 1) ← equation of variation
When c = 20, then
20 = 2(r + 1) ← divide both sides by 2
10 = r + 1 ( subtract 1 from both sides )
9 = r
c = k(r + 1)
8 = k(3 + 1)
8 = 4k
8/4 = k
2 = k
c=2(r + 1)
When c = 20, then
20 = 2(r + 1)
20 = 2r + 2
20 - 2 = 2r
18 = 2r
18/2 = r
9 = r
Therefore, the value of r is 9 when c is 20..
