Cost of fruit combination

Let $o, b, a$ be the costs of an orange, banana, and apple respectively.

$2o + 3b + 4a = 15 \quad \text{--- (1)}$ $3o + 2b + a = 10 \quad \text{--- (2)}$

Adding equations (1) and (2): $5o + 5b + 5a = 25 \implies o + b + a = 5$

Cost of $3$ oranges, $3$ bananas, and $3$ apples: $3(o + b + a) = 3 \times 5 = \text{Rs. } 15$