To solve this equation, we need to simplify each term and then combine like terms.

Starting with the first term, a × 6/50, we can simplify it by multiplying a by 6 and then dividing by 50:

a × 6/50 = 6a/50 = 3a/25

Next, let's simplify the second term, a: 5/42. Since there is a colon between a and 5/42, we can assume it means multiplication:

a × 5/42 = 5a/42

Moving on to the third term, a + a, we can combine the two a's:

a + a = 2a

Finally, let's simplify the fourth term, a: 5/3. Again, assuming the colon means multiplication:

a × 5/3 = 5a/3

Now, let's rewrite the equation with the simplified terms:

3a/25 + 5a/42 + 2a + 5a/3 = 28

To combine the fractions, we need to find a common denominator. The least common multiple of 25, 42, and 3 is 1050. Let's rewrite each fraction with the common denominator:

(3a/25) × (42/42) = 126a/1050
(5a/42) × (25/25) = 125a/1050
(5a/3) × (350/350) = 1750a/1050

Now, the equation becomes:

(126a/1050) + (125a/1050) + 2a + (1750a/1050) = 28

Combining the fractions:

(126a + 125a + 2a + 1750a) / 1050 = 28

Simplifying the numerator:

(2003a) / 1050 = 28

To isolate a, we can multiply both sides of the equation by 1050/2003:

a = (28 × 1050) / 2003

Evaluating the right side of the equation:

a ≈ 14.7