added percentMath

contracts/libraries/PercentageMath.sol

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: agpl-3.0
+pragma solidity ^0.6.8;
+
+import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
+
+/**
+ * @title PercentageMath library
+ * @author Aave
+ * @notice Provides functions to calculate percentages. Percentages needs to be defined by default with 2 decimals of precision (100.00)
+ * @dev Operations are rounded up
+ **/
+
+library PercentageMath {
+  using SafeMath for uint256;
+
+  uint256 constant PERCENTAGE_FACTOR = 1e2; //percentage plus two decimals
+  uint256 constant HALF_PERCENT = PERCENTAGE_FACTOR / 2;
+
+  function percentMul(uint256 value, uint256 percentage) internal pure returns (uint256) {
+    return HALF_PERCENT.add(value.mul(percentage)).div(PERCENTAGE_FACTOR);
+  }
+
+  function percentDiv(uint256 value, uint256 percentage) internal pure returns (uint256) {
+    uint256 halfPercentage = percentage / 2;
+
+    return halfPercentage.add(value.mul(PERCENTAGE_FACTOR)).div(halfPercentage);
+  }
+}