Move the decimal point one place to the right, or add a zero.

Example:

If the 2-digit number chosen to multiply by 2 1/2
is 98

Divide by 2: 98/2 = 49

Divide by 2: 49/2 = 24.5.

Move the decimal point one place to the right: 245

So 98 × 2 1/2 = 245.

See the pattern?

If the 3-digit number chosen to multiply by 2 1/2
is 123

Divide by 2: 123/2 = 61.5

Divide by 2: 61.5/2 = 30.75.

Move the decimal point one place to the right: 307.5

So 123 × 2 1/2 = 307.5.

If the 3-digit number chosen to multiply by 2 1/2
is 488

Divide by 4: 488/4 = 122

Divide by 2: 61.5/2 = 30.75.

Move the decimal point one place to the right (add a zero): 1220.

So 488 × 2 1/2 = 1220.

Divide by 4 when it is easy (the number is divisible by 4 if its last two digits
are divisible by 4), or divide by 2 twice. This method should become an easy one.