Finding the start

In Primary 2, you learnt that subtracting undoes adding, and adding undoes subtracting. Today, we work backwards to find how many there were at first. A science centre had some brochures. It gave out one hundred and twenty-five brochures, then received forty-eight more. It had three hundred and seventy-six brochures at the end. How many brochures did it have at first? We must find how many brochures it had at first. We know how many it had at the end, so we work backwards from the last change. Last, it received forty-eight, so we subtract forty-eight to undo it. Three hundred and seventy-six minus forty-eight is three hundred and twenty-eight. Before it received forty-eight, it had three hundred and twenty-eight brochures. First, it gave out one hundred and twenty-five, so we add one hundred and twenty-five back. Three hundred and twenty-eight plus one hundred and twenty-five is four hundred and fifty-three. The science centre had four hundred and fifty-three brochures at first.

A science centre had some brochures. It gave out two hundred and fifteen brochures, then received sixty-four more. It had one thousand, two hundred and eighty-six brochures at the end. How many brochures did it have at first? Try it first. Pause if you like. We must find how many brochures it had at first. We know how many it had at the end, so we work backwards from the last change. Last, it received sixty-four, so we subtract sixty-four to undo it. One thousand, two hundred and eighty-six minus sixty-four is one thousand, two hundred and twenty-two. Before it received sixty-four, it had one thousand, two hundred and twenty-two brochures. First, it gave out two hundred and fifteen, so we add two hundred and fifteen back. One thousand, two hundred and twenty-two plus two hundred and fifteen is one thousand, four hundred and thirty-seven. The science centre had one thousand, four hundred and thirty-seven brochures at first.

A science centre had some activity cards. It received one hundred and thirty-four activity cards, then gave out eighty-seven. It had five hundred and sixty-two activity cards at the end. How many activity cards did it have at first? Try it first. Pause if you like. We must find how many activity cards it had at first. We know how many it had at the end, so we work backwards from the last change. Last, it gave out eighty-seven, so we add eighty-seven back to undo it. Five hundred and sixty-two plus eighty-seven is six hundred and forty-nine. Before it gave out eighty-seven, it had six hundred and forty-nine activity cards. First, it received one hundred and thirty-four, so we subtract one hundred and thirty-four. Six hundred and forty-nine minus one hundred and thirty-four is five hundred and fifteen. The science centre had five hundred and fifteen activity cards at first. To find how many there were at first, work backwards and undo the last change first.