A number when divided by 21 leaves a remainder 3. When thrice the number is divided by 7, the remainder isa.2b.1c.0d.3

Let's denote the number as x.

Step 1: According to the problem, when x is divided by 21, the remainder is 3. This can be written as: x = 21k + 3, where k is the quotient.

Step 2: The problem also states that when thrice the number (3x) is divided by 7, we need to find the remainder. Substituting x from step 1, we get: 3x = 3(21k + 3) = 63k + 9.

Step 3: Now, we need to find the remainder when 63k + 9 is divided by 7. Since 63k is divisible by 7, we only need to consider the remainder when 9 is divided by 7.

Step 4: When 9 is divided by 7, the remainder is 2.

So, the answer is a.2.