B =4:2/5=4*2/5

Short answer: the step you wrote isn’t correct if you’re using the standard meaning of a:b as a divided by b.

• Generally, 4:2/5 means 4 ÷ (2/5) = 4 × (5/2) = 10.
• Your reduction 4:2/5 = 4 × 2/5 would give 8/5, which is not equal to 10.

For clarity, check how the expression is meant to be read:

• If it’s 4 ÷ (2/5) → 10.
• If someone mistakenly means (4 ÷ 2) ÷ 5 → (2) ÷ 5 = 2/5 = 0.4.
• If you meant 4 × (2/5) → 8/5 = 1.6 (not a division-by-b) either.

If you can share the exact context or how the problem is written (with parentheses), I can confirm the intended interpretation.