Let, Aruna buys 'a' number of apples and 'm' number of mangoes
So, total C.P. =$20a+25m$
In the first case, whenshe sells all the apples at 10% profit and all the mangoes at 20% loss, total S.P.= $20a\times\ 1.1+25m\times\ 0.8$
In this case, she is neither making profit nor loss.
So, total C.P. = total S.P.
or, $20a+25m=20a\times\ 1.1+25m\times\ 0.8$
or,$20a+25m=22a+20m$
so,$2a=5m$ ----(1)
In the second case,whenshe sells all the apples at 20% loss and all the mangoes at 10% profit, total S.P. =$20a\times\ 0.8+25m\times\ 1.1$
or, Total S.P. =$16a+27.5m$
In this case, she made a loss of INR 150.
So, Total S.P.+150 = Total C.P.
or,$16a+27.5m+150=20a+25m$
or,$2.5m+150=4a$ -----(2)
Now, from equation (1),
$2a=5m$or,$a=2.5m$
Putting this in equation (2),$a+150=4a$
or,$3a=150$
or,$a=\dfrac{150}{3}=50$
So, number of apples purchased by Aruna is 50