One quantity unchanged

In an earlier lesson, we drew bars of equal units and found one unit. Today, we will find whose shells stay the same, and use them as 1 unit before and after the change. Mei had 3 times as many shells as Kavin. Mei gave away 14 shells. Then Mei had the same number of shells as Kavin. How many shells did Mei have at first? Mei is compared with Kavin, so Kavin's shells make 1 unit. Kavin did not give any shells away, so his 1 unit stays the same. At first, Mei had 3 times as many shells as Kavin, so her bar is 3 units. Mei gave away 14 shells. Then Mei had the same number as Kavin, so she had 1 unit. These are the 14 shells Mei gave away. The question asks how many shells Mei had at first. 3 units minus 1 unit is 2 units. These 2 units are 14 shells. To find 1 unit, we share 14 equally between the 2 units. 14 divided by 2 is 7. Mei had 3 units at first. 3 times 7 is 21. So Mei had 21 shells at first.

Siti had 4 times as many shells as Bala. Siti gave away 18 shells. Then Siti had 2 times as many shells as Bala. How many shells did Siti have at first? Try it first. Pause if you like. Siti is compared with Bala, so Bala's shells make 1 unit. Bala did not give any shells away, so his 1 unit stays the same. At first, Siti had 4 times as many shells as Bala, so her bar is 4 units. Siti gave away 18 shells. Then Siti had 2 times as many as Bala, so she had 2 units. These are the 18 shells Siti gave away. The question asks how many shells Siti had at first. 4 units minus 2 units is 2 units. These 2 units are 18 shells. To find 1 unit, we share 18 equally between the 2 units. 18 divided by 2 is 9. Siti had 4 units at first. 4 times 9 is 36. So Siti had 36 shells at first.

Ravi had 2 times as many shells as Wei Jie. Then Ravi found 24 more shells. Now Ravi has 5 times as many shells as Wei Jie. How many shells does Wei Jie have? Try it first. Pause if you like. Ravi is compared with Wei Jie, so Wei Jie's shells make 1 unit. Wei Jie did not find any more shells, so his 1 unit stays the same. At first, Ravi had 2 units. Ravi found 24 more shells, so his bar grows to 5 units. Ravi found 24 more shells. The question asks how many shells Wei Jie has. 5 units minus 2 units is 3 units. These 3 units are 24 shells. Wei Jie has just 1 unit. To find 1 unit, we share 24 equally among the 3 units. 24 divided by 3 is 8. So Wei Jie has 8 shells. Find whose shells stay the same. Their bar is 1 unit, before and after the change.