Mr. brown owned a house a house, which he rented for $60 a month. the house was assessed for $9000. in 1975 the rate of taxation was increased from $25 to $28 per $1000 assessed valuation. by what amount should the monthly rent have been raised to absorb the increase in that year's taxes?

We first calculate the percentage increase on the tax

Old value = 25×9 = 225 ⇒ The value 9 represents the 9 lots of thousands of the house's value
New value = 28×9 = 252

Increase in tax = 252 - 225 =27  
Percentage increase = (27÷225) ×100 = 12%

The amount of yearly rent would be then increased by 12%

Monthly rent = $60
Yearly rent = 60×12 = $720
Increase by 12% = 720×1.12 = 806.4 ⇒ The value 1.12 is the multiplier, obtained from 100%+12%=112%=1.12

The monthly rent is 806.4÷12 = $67.20 which is an increase of $7.20 per month

How many groups are there?