Which term of the progression 0.004,0.02,0.1,… is 12.5?

Explanation

Let ` a_(n) = 12.5 `
` rArr ar^(n-1) = 12.5 `
` rArr ((4)/(1000)) xx (5)^(n-1) = 12.5 `
` rArr 5^(n-1) = (12500)/(4) = 125 xx25 `
` rArr 5^(n-1) = (5)^(5)`
` therefore n-1 = 5 `
`rArr n=6 ` .