For Q7, we treat d as any other number, except that it is a "mystery" number.

(a) Total spending
= 4 Pencils + 1 Pen + 1 Eraser
= 4 x 2d + (d + 85) + (d - 15)
= 8 x d + d + 85 + d - 15
= 8d + d + d + 85 - 15
= (10d + 70) cents

(b) Cost of pen
= d + 85
= 25 + 85
= 110 cents

Cost of eraser
= d - 15
= 25 - 15
= 10 cents

Difference in cost of pen and eraser
= 110 - 10
= 100 cents
= $1

Q8

Let the top of the triangle be called R

The angle next to the 104 degrees is k, since RPQ = RBC = k before it is folded inwards (we call this corresponding angles)

For the right side figure, the top left angle is 104 + k.

The bottom left angle is k.

These two add up to 180 degrees (the sum of angles in such manner, I call it as interior angles between parallel lines, is 180 degrees)

So,

k + (104 + k) = 180
k + 104 + k = 180
k + k + 104 = 180
2k + 104 = 180

Subtracting 104 from both sides,
2k + 104 - 104 = 180 - 104
2k + 0 = 76
2k = 76

Dividing both sides by 2,
2k/2 = 76/2
1k = 38
k = 38