you are dividing too early. 2o=3a or 2o+2a=5a so 60=5a and a=12 thus o=18. the other way 2o=3a or 5o=3a+3o=90 and o=18. keep items in integers (units are cents) and the division is easier in the head. no calculator or paper/pen needed.
you are dividing too early. 2o=3a or 2o+2a=5a so 60=5a and a=12 thus o=18. the other way 2o=3a or 5o=3a+3o=90 and o=18. keep items in integers (units are cents) and the division is easier in the head. no calculator or paper/pen needed.