Step 1: Set variables and translate the statements.
Let the unit prices be: apple $=a$, mango $=m$, banana $=b$.
Let the numbers purchased be: apples $=A$, mangoes $=M$, bananas $=B$.
Given $M=B$ (same number of mangoes and bananas). Let $M=B=n$.
Step 2: "If mangoes cost as apples, bananas must be foregone."
With the hypothetical $m=a$, and keeping the same total spend (₹ 140), the cost of \emph{only} apples and mangoes uses the full amount:
\[
Aa + Ma = 140.
\]
But in reality,
\[
Aa + Mm + Bb = 140.
\]
Equating the two totals gives
\[
Ma = Mm + Bb \;\Rightarrow\; M(a-m)=Bb.
\]
Since $M=B=n$, this simplifies to
\[
a - m = b. \qquad (1)
\]
Step 3: "Mangoes + Bananas cost is 50\% more than Apples."
\[
Mm + Bb = 1.5\,(Aa).
\]
Using $M=B=n$ and \((1)\Rightarrow m+b=a\), we obtain
\[
n(m+b) = n a = 1.5\,Aa \;\Rightarrow\; n = 1.5\,A = \frac{3A}{2}. \qquad (2)
\]
Step 4: Use the total bill ₹ 140 to find $Aa$.
\[
140 = Aa + n(m+b) = Aa + n a \quad (\text{since } m+b=a).
\]
Thus
\[
140 = (A+n)a = \Big(A + \tfrac{3A}{2}\Big)a = \tfrac{5A}{2}\,a
\;\Rightarrow\; Aa = 140\cdot \tfrac{2}{5} = 56. \qquad (3)
\]
Step 5: Extra money to make apples as many as mangoes/bananas.
Target apples $=n=\tfrac{3A}{2}$, so extra apples needed $= n-A = \tfrac{A}{2}$.
Additional cost $= \tfrac{A}{2}\cdot a = \tfrac{1}{2}(Aa) = \tfrac{1}{2}\times 56 = \boxed{28}.
$