More than half?

40×85÷100=34

85-34=51

Using mental math:

40% used means that 60% remains. $60/100 \times 85 = 0.6 \times 85 = 6 \times 8.5 = 6 \times 8 + 6 \times 0.5 = 48 + 3$

$\frac{60}{100} \times 85= 51$

Why do so many people think useful to add yet another explanation for a substraction followed by a multiplication (or vice-versa)?

It is simple.

60% of tank would be there.

100%=85, 40%=×, When you cross multiply, 100x =40 × 85, x = (40 × 85)/100, x = 34, x is the amount used. To find the amount left, subtract 34 from 85. Therefore, the answer is 51.

100% - 40% = 60% used 60% x 85 = 51

85×.40=34. 85-34=51

(60*85)/100

water used = 40/100 * 85 = 34 liters water remained = 85 - 34 = 51 liters so simple ....

85×.40=34 then 85-34=51

(100-40)% of 85 L = 0.6 X 85 L = 51 L

Maybe not elegant, but quick mental math: 85 x 0.10= 8.5 8.5 x 2= 17 17 x 2= 34 85 - 34= 51

Basically, found 10% of 85, multiplied by 4 (well, 2 and then 2 again) to get 40% and subtracted the result from 85.

60/100*85=51

85 - (85)(40/100) = 85 - (3/5)85 = 85 - 34 = 51

The solution given are correct but let me give a detailed one. Given that 40 litres out of 100 litres are used therefore 60 litres are left.

Similary if 60 litres out of 100 litres are left therefore out of 85 litres $\frac{60}{100}\times 85 = 51$

40/100*85=34 85-34=51