.
Eli Al’s has a collection of marbles. If he gives away 3/5 of his marbles to his friend
and then buys 20 more marbles, he will have 100 marbles in total.
How many marbles did Eli Al have originally?
~~~~~~~~~~~~~~~~~~~~~~
Good problem to solve it MENTALLY by a BACKWARD method.
+----------------------------------------------------------------+
| We start from the end and move backward to the beginning |
| step by step, using reasoning and making simple arithmetic. |
| No equations are needed, so this method is good even for |
| 4-th grade young students of primary schools. |
+----------------------------------------------------------------+
At the end, Eli Al has 100 marbles.
Hence, immediately before he bought 20 marbles, he had 100-20 = 80 marbles.
Hence, these 80 marbles were = of marbles he had initially.
Hence, initially he had = 5*40 = 200 marbles.
ANSWER. Initially, Eli Al had 200 marbles.
Solved.
---------------------
To see many other similar and different solved problems, look into the lesson
Simple arithmetic word problems to solve by a backward method
in this site.
Learn the method from there.