Mixed Number to an Improper Fraction

LabVIEW, 29 LabVIEW Primitives

CJam, 16 15 14 bytes

l'/']er~:Xb'/X

or

l'/']er~_@b'/@

Test it here.

Explanation

l      e# Read input.
'/']er e# Replace the "/" with a "]".
~      e# Evaluate the string as code. If the input was "i n/d", this pushes [i n] d.
:X     e# Store the denominator in X.
b      e# Treat [i n] as base-d digits. This effectively computes i*d + n.
'/     e# Push a slash.
X      e# Push the denominator.

The other version avoids using a variable by using a bit more stack shifting.

Mathematica, 58 bytes

ToExpression@StringReplace[#," "->"+"]~ToString~InputForm&

This returns the simplified result. If outputting a rational number instead of a string is fine, we can save 19 bytes:

ToExpression@StringReplace[#," "->"+"]&