Mrs Lim packs 35 big and small bags of candies for her pupils. Each big bag contains 12 candies and each small bag contains 7 candies. The bags contain 300 candies altogether. How many big bags of candies and how many small bags of candies does she pack?

there are 11 big-bags and 24 small-bags.

Step-by-step explanation:

there are x big-bags and y small-bags.

so now we can know:

(1) x + y = 35

(2) 12x + 7y = 300

in (1), we can do like this:

both left and right x7

then (x + y) x7 = 35 x 7

then 7x + 7y = 35 x 7

then 7x + 7y = 245

now,

(1) 7x + 7y = 245

(2) 12x + 7y = 300

we can both left and right do this: (2) - (1)

then

(12x + 7y) - (7x + 7y) = 300 - 245

then

12x + 7y - 7x - 7y = 300 - 245

then

12x - 7x + 7y - 7y = 300 - 245

then

5x = 55

then

5x : 5 = 55 : 5

then

x = 11

because

x + y = 35; x=11

so

11 + y = 35

11 + y - 11 = 35 - 11

then

y = 24

now we know:there are 11 big-bags and 24 small-bags.