I'll give you a good rating if you answer this:

Here the first number after 50 divisible by 3 is  51. But that is odd.

The next number divisible by 3 is 54.

Hence the first term is 54.

Then the numbers increase by a common difference 6.

The last even number divisible by 3 less than 100 is 96.

So

96 = 54 + (n-1) 6

96-54 = (n-1)6

42 = (n-1)6

42/6= n-1

7 = n-1

7+1 = n

n = 8    ......Answer

(Method 1)There are 10 multiples of 3 in 1- 30, So there are 10 multiples of 3 in 50 - 80. Then we count from 81.

81, 84, 87, 90, 93, 96, 99.

There are 17 multiples of 3 between 50 and 100.

Then we have 99 is not an even number.

51, 54, 57, 60..... , 96 <---- 16 numbers there.

Then we divide 16 by 2 which is 8

There are 8 even multiples of 3 between 50 and 100.

(Method 2) Even multiples of 3 must be divisible by 6 because those multiples of 3 are divisible by 2 and 3.

Starting from 54, we have

54, 60, 66, 72, 78, 84, 90, 96 Which is 8 numbers.