PSLE
Harry packed 358 wafers into large and small boxes. He filled each large box with 14 wafers and each small box with 10 wafers. All the boxes were full and no wafers were left over. What was the least number of boxes used by Harry?
To have the least number of boxes, Harry had to use as many as large boxes as possible.

If Harry used only large boxes,
number of large boxes used
= 358 ÷ 14
= 25 r 8 (x) There should be no remaining wafers left.

If 25 large boxes are used, there are 8 wafers left, Harry had to use less large boxes so that the rest of the wafers were packed in small boxes.

If Harry used 24 large boxes,
number of wafers to be packed in small boxes
= 358 - (24 x 14)
= 358 - 336
= 22

Number of small boxes needed to pack 22 wafers
= 22 ÷ 10
= 2 r 2 (x)

There should be no remaining wafers left. So, Harry had to use less large boxes.

If Harry used 23 large boxes,
number of wafers to be packed in small boxes
= 358 - (23 x 14)
= 358 - 322
= 36

Number of small boxes to pack 36 wafers
= 36 ÷ 10
= 3 r 6 (x)

There should be no remaining wafers left. So, Harry had to use less large boxes.

If Harry used 22 large boxes,
number of wafers to be packed in small boxes
= 358 - (22 x 14)
= 358 - 308
= 50

Number of small boxes to pack 50 wafers
= 50 ÷ 10
= 5 r 0 (✓) There is no remaining wafers left.

Least number of boxes that Harry used
= 22 + 5
= 27

Answer(s): 27