Sandra's dog weighs 3 1/2 pounds. Each dog he gains 2/3 of a pound. If he continues to gain this much weight each week, how much will he weigh in 8?

Multiply the weight gain per week by the number of weeks, then add it to the staring weight.

2/3 lb x 8 week = 16/3 = 5 1/3 pounds.

Since both fractions have different denominators, convert the weights to improper fractions, then turn them into fractions with a common denominator then simplify:

3 1/2 = 7/2

5 1/3 = 16/3

Common denominator is 6:

7/2 * 3 = 21/6

16/3 x 2 = 32/6

Now add:

21/6 + 32/6 = 53/6

No convert back to proper fraction:

53/6 = 8 5/6 pounds.