Para embalar una caja se emplea 4,2 m de cinta adhesiva. ? cuántas cajas se podrán embalar con tres rollos que tienen 3 hm, 7 dam y 50 m cada uno?

For this case, we perform the conversions:

First roll:

We make a rule of three to determine the number of "c" boxes that can be packed with 300 meters of adhesive tape.

1 -----------> 4.2

c -----------> 300

You can pack 71 boxes.

Second roll:

We make a rule of three to determine the number of "c" boxes that can be packed with 70 meters of adhesive tape.

1 -----------> 4.2

c -----------> 70

You can pack 16 boxes.

Third roll:

1 -----------> 4.2

c -----------> 50

You can pack 11 boxes.

Thus, in total you can pack

98 boxes

the answer is y = 11.54

hope this : )