目からうろこです。
Sub AddFractions()
'定数の宣言
Dim numer1 As Double, denom1 As Double, numer2 As Double, denom2 As Double
Dim numer3 As Double, denom3 As Double, result As String
'分子と分母を取得
numer1 = Range("A1").Value
denom1 = Range("A2").Value
numer2 = Range("B1").Value
denom2 = Range("B2").Value
'最終分母を計算
denom3 = denom1 * denom2
'最終分子を計算
numer3 = (numer1 * denom2) + (numer2 * denom1)
'分数として表示
result = numer3 & "/" & denom3
'結果を出力
Range("C1").Value = result
End Sub